diff --git a/substrate/primitives/src/block.rs b/substrate/primitives/src/block.rs
index dd297535c5..8e36366f2c 100644
--- a/substrate/primitives/src/block.rs
+++ b/substrate/primitives/src/block.rs
@@ -16,6 +16,7 @@
 
 //! Block and header type definitions.
 
+use bytes;
 use hash::H256;
 
 /// Hash used to refer to a block hash.
@@ -37,7 +38,7 @@ impl From<u64> for ParachainId {
 
 /// A wrapper type for parachain block header raw bytes.
 #[derive(Debug, PartialEq, Eq, Serialize, Deserialize)]
-pub struct ParachainBlockHeader(pub Vec<u8>);
+pub struct ParachainBlockHeader(#[serde(with="bytes")] pub Vec<u8>);
 
 /// A parachain block proposal.
 #[derive(Debug, PartialEq, Eq, Serialize, Deserialize)]
@@ -108,12 +109,7 @@ mod tests {
   "para_blocks": [
     {
       "parachain": 5,
-      "header": [
-        1,
-        2,
-        3,
-        4
-      ],
+      "header": "0x01020304",
       "proof_hash": "0x0000000000000000000000000000000000000000000000000000000000000005"
     }
   ]
diff --git a/substrate/primitives/src/bytes.rs b/substrate/primitives/src/bytes.rs
new file mode 100644
index 0000000000..efe21da706
--- /dev/null
+++ b/substrate/primitives/src/bytes.rs
@@ -0,0 +1,99 @@
+// Copyright 2017 Parity Technologies (UK) Ltd.
+// This file is part of Polkadot.
+
+// Polkadot is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+
+// Polkadot is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License
+// along with Polkadot.  If not, see <http://www.gnu.org/licenses/>.
+
+use std::fmt;
+
+use serde::{de, Serializer, Deserializer};
+
+/// Serializes a slice of bytes into a most compact form.
+pub fn serialize<S>(bytes: &[u8], serializer: S) -> Result<S::Ok, S::Error> where
+	S: Serializer,
+{
+	let hex = ::rustc_hex::ToHex::to_hex(bytes);
+	serializer.serialize_str(&format!("0x{}", hex))
+}
+
+pub fn serialize_compact<S>(bytes: &[u8], serializer: S) -> Result<S::Ok, S::Error> where
+	S: Serializer,
+{
+	let mut non_zero = bytes.len();
+	for (i, b) in bytes.iter().enumerate() {
+		if *b != 0 {
+			non_zero = i;
+			break;
+		}
+	}
+
+	if non_zero == bytes.len() {
+		return serializer.serialize_str("0x0");
+	}
+
+	let hex = ::rustc_hex::ToHex::to_hex(&bytes[non_zero..]);
+	let has_leading_zero = !hex.is_empty() && &hex[0..1] == "0";
+	return serializer.serialize_str(&format!("0x{}", if has_leading_zero { &hex[1..] } else { &hex }));
+}
+
+pub fn deserialize<'de, D>(deserializer: D) -> Result<Vec<u8>, D::Error> where
+	D: Deserializer<'de>,
+{
+	deserialize_with_check(deserializer, |_| Ok(()))
+}
+
+pub fn deserialize_with_check<'de, D, F>(deserializer: D, check: F) -> Result<Vec<u8>, D::Error> where
+	D: Deserializer<'de>,
+	F: Fn(&str) -> Result<(), ErrorKind>,
+{
+	struct Visitor<F> {
+		check: F,
+	}
+
+	impl<'a, F> de::Visitor<'a> for Visitor<F> where
+		F: Fn(&str) -> Result<(), ErrorKind>,
+	{
+		type Value = Vec<u8>;
+
+		fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+			write!(formatter, "a 0x-prefixed hex string")
+		}
+
+		fn visit_str<E: de::Error>(self, v: &str) -> Result<Self::Value, E> {
+			if v.len() < 2  || &v[0..2] != "0x" {
+				return Err(E::custom("prefix is missing"))
+			}
+
+			(self.check)(v).map_err(|err| match err {
+				ErrorKind::InvalidLength(len) => E::invalid_length(len, &self),
+			})?;
+
+			let v = if v.len() % 2 == 0 { v[2..].to_owned() } else { format!("0{}", &v[2..]) };
+			let bytes = ::rustc_hex::FromHex::from_hex(v.as_str())
+				.map_err(|e| E::custom(&format!("invalid hex value: {:?}", e)))?;
+			Ok(bytes)
+		}
+
+		fn visit_string<E: de::Error>(self, v: String) -> Result<Self::Value, E> {
+			self.visit_str(&v)
+		}
+	}
+	// TODO [ToDr] Use raw bytes if we switch to RLP / binencoding
+	// (visit_bytes, visit_bytes_buf)
+	deserializer.deserialize_str(Visitor { check })
+}
+
+pub enum ErrorKind {
+	InvalidLength(usize),
+}
+
diff --git a/substrate/primitives/src/hash.rs b/substrate/primitives/src/hash.rs
index 788a395f0e..a93324276e 100644
--- a/substrate/primitives/src/hash.rs
+++ b/substrate/primitives/src/hash.rs
@@ -16,52 +16,28 @@
 
 //! A fixed hash type.
 
-use std::fmt;
-use serde::{de, Serialize, Serializer, Deserialize, Deserializer};
+use serde::{Serialize, Serializer, Deserialize, Deserializer};
+
+use bytes;
 
 macro_rules! impl_serde {
 	($name: ident, $len: expr) => {
 		impl Serialize for $name {
 			fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error> where S: Serializer {
-				// TODO [ToDr] Use raw bytes if we switch to RLP / binencoding
-				// (serialize_bytes)
-				serializer.serialize_str(&format!("0x{:?}", self))
-
+				bytes::serialize(&self.0, serializer)
 			}
 		}
 
 		impl<'de> Deserialize<'de> for $name {
 			fn deserialize<D>(deserializer: D) -> Result<Self, D::Error> where D: Deserializer<'de> {
-				struct Visitor;
-				impl<'a> de::Visitor<'a> for Visitor {
-					type Value = $name;
-
-					fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
-						write!(formatter, "a 0x-prefixed hex string of {} bytes", $len)
+				bytes::deserialize_with_check(deserializer, |v: &str| {
+					// 0x + len
+					if v.len() != 2 + $len * 2 {
+						Err(bytes::ErrorKind::InvalidLength(v.len() - 2))
+					} else {
+						Ok(())
 					}
-
-					fn visit_str<E: de::Error>(self, v: &str) -> Result<Self::Value, E> {
-						if v.len() < 2  || &v[0..2] != "0x" {
-							return Err(E::custom("prefix is missing"))
-						}
-
-						// 0x + len
-						if v.len() != 2 + $len * 2 {
-							return Err(E::invalid_length(v.len() - 2, &self));
-						}
-
-						let bytes = ::rustc_hex::FromHex::from_hex(&v[2..])
-							.map_err(|e| E::custom(&format!("invalid hex value: {:?}", e)))?;
-						Ok((&*bytes).into())
-					}
-
-					fn visit_string<E: de::Error>(self, v: String) -> Result<Self::Value, E> {
-						self.visit_str(&v)
-					}
-				}
-				// TODO [ToDr] Use raw bytes if we switch to RLP / binencoding
-				// (visit_bytes, visit_bytes_buf)
-				deserializer.deserialize_str(Visitor)
+				}).map(|x| (&*x).into())
 			}
 		}
 	}
diff --git a/substrate/primitives/src/lib.rs b/substrate/primitives/src/lib.rs
index a4a5337744..596c13f4ac 100644
--- a/substrate/primitives/src/lib.rs
+++ b/substrate/primitives/src/lib.rs
@@ -38,6 +38,7 @@ extern crate polkadot_serializer;
 #[macro_use]
 extern crate pretty_assertions;
 
+mod bytes;
 pub mod block;
 pub mod hash;
 pub mod uint;
diff --git a/substrate/primitives/src/uint.rs b/substrate/primitives/src/uint.rs
index 305523fe71..8f1c07bde1 100644
--- a/substrate/primitives/src/uint.rs
+++ b/substrate/primitives/src/uint.rs
@@ -16,57 +16,30 @@
 
 //! An unsigned fixed-size integer.
 
-use std::fmt;
-use serde::{de, Serialize, Serializer, Deserialize, Deserializer};
+use serde::{Serialize, Serializer, Deserialize, Deserializer};
+
+use bytes;
 
 macro_rules! impl_serde {
 	($name: ident, $len: expr) => {
 		impl Serialize for $name {
 			fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error> where S: Serializer {
-				// TODO [ToDr] Use raw bytes if we switch to RLP / binencoding
-				// (serialize_bytes)
-				if self.is_zero() {
-					// TODO [ToDr] LowerHex of 0 is broken
-					serializer.serialize_str("0x0")
-				} else {
-					serializer.serialize_str(&format!("{:#x}", self))
-				}
+				let mut bytes = [0u8; $len * 8];
+				self.to_big_endian(&mut bytes);
+				bytes::serialize_compact(&bytes, serializer)
 			}
 		}
 
 		impl<'de> Deserialize<'de> for $name {
 			fn deserialize<D>(deserializer: D) -> Result<Self, D::Error> where D: Deserializer<'de> {
-				struct Visitor;
-				impl<'a> de::Visitor<'a> for Visitor {
-					type Value = $name;
-
-					fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
-						write!(formatter, "a 0x-prefixed hex string")
+				bytes::deserialize_with_check(deserializer, |v: &str| {
+					// 0x + len
+					if v.len() > 2 + $len * 16 || v.len() == 2 {
+						Err(bytes::ErrorKind::InvalidLength(v.len() - 2))
+					} else {
+						Ok(())
 					}
-
-					fn visit_str<E: de::Error>(self, v: &str) -> Result<Self::Value, E> {
-						if v.len() < 3  || &v[0..2] != "0x" {
-							return Err(E::custom("prefix is missing"))
-						}
-
-						// 0x + len
-						if v.len() > 2 + $len * 16 {
-							return Err(E::invalid_length(v.len() - 2, &self));
-						}
-
-						let v = if v.len() % 2 == 0 { v[2..].to_owned() } else { format!("0{}", &v[2..]) };
-						let bytes = ::rustc_hex::FromHex::from_hex(v.as_str())
-							.map_err(|e| E::custom(&format!("invalid hex value: {:?}", e)))?;
-						Ok((&*bytes).into())
-					}
-
-					fn visit_string<E: de::Error>(self, v: String) -> Result<Self::Value, E> {
-						self.visit_str(&v)
-					}
-				}
-				// TODO [ToDr] Use raw bytes if we switch to RLP / binencoding
-				// (visit_bytes, visit_bytes_buf)
-				deserializer.deserialize_str(Visitor)
+				}).map(|bytes| (&*bytes).into())
 			}
 		}
 	}