diff --git a/src/extrinsic/mod.rs b/src/extrinsic/mod.rs
index d4c175792a..c3b90feac0 100644
--- a/src/extrinsic/mod.rs
+++ b/src/extrinsic/mod.rs
@@ -17,6 +17,7 @@
 //! Create signed or unsigned extrinsics.
 
 mod extra;
+mod mortal_period;
 mod signer;
 
 pub use self::{
@@ -25,6 +26,7 @@ pub use self::{
         Extra,
         SignedExtra,
     },
+    mortal_period::derive_mortal_period,
     signer::{
         PairSigner,
         Signer,
@@ -44,9 +46,6 @@ use crate::{
     Error,
 };
 
-/// A reasonable default for `mortal_period`
-pub const DEFAULT_MORTAL_PERIOD: u64 = 64;
-
 /// UncheckedExtrinsic type.
 pub type UncheckedExtrinsic<T> = sp_runtime::generic::UncheckedExtrinsic<
     <T as System>::Address,
diff --git a/src/extrinsic/mortal_period.rs b/src/extrinsic/mortal_period.rs
new file mode 100644
index 0000000000..115705438a
--- /dev/null
+++ b/src/extrinsic/mortal_period.rs
@@ -0,0 +1,55 @@
+// Copyright 2019-2020 Parity Technologies (UK) Ltd.
+// This file is part of substrate-subxt.
+//
+// subxt 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.
+//
+// subxt 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 substrate-subxt.  If not, see <http://www.gnu.org/licenses/>.
+
+use crate::{
+    Error,
+    Metadata,
+};
+
+/// 5 min `mortal_period` in milliseconds, to be adjusted based on expected block time
+pub const BASELINE_MORTAL_PERIOD: u64 = 5 * 60 * 1000;
+
+/// Fallback `BlockHashCount` of 2,400 blocks
+pub const FALLBACK_BLOCK_HASH_COUNT: u64 = 2_400;
+
+/// Fallback expected block time of 6,000 milliseconds
+pub const FALLBACK_EXPECTED_BLOCK_TIME: u64 = 6_000;
+
+/// Derive a default mortal period based on a chain's metadata
+pub fn derive_mortal_period(metadata: &Metadata) -> Result<u64, Error> {
+    let block_hash_count = if let Ok(system_meta) = metadata.module("System") {
+        if let Ok(count) = system_meta.constant("BlockHashCount") {
+            count.value::<u32>()?.into()
+        } else {
+            FALLBACK_BLOCK_HASH_COUNT
+        }
+    } else {
+        FALLBACK_BLOCK_HASH_COUNT
+    };
+    let block_time = if let Ok(babe_meta) = metadata.module("Babe") {
+        if let Ok(milliseconds) = babe_meta.constant("ExpectedBlockTime") {
+            milliseconds.value::<u64>()?
+        } else {
+            FALLBACK_EXPECTED_BLOCK_TIME
+        }
+    } else {
+        FALLBACK_EXPECTED_BLOCK_TIME
+    };
+
+    Ok((BASELINE_MORTAL_PERIOD / block_time)
+        .next_power_of_two()
+        .min(block_hash_count.next_power_of_two()))
+}
diff --git a/src/lib.rs b/src/lib.rs
index b775680af3..b69fead1ff 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -50,6 +50,7 @@ pub use sp_core;
 pub use sp_runtime;
 
 use codec::Decode;
+use core::convert::Into;
 use futures::future;
 use jsonrpsee::client::Subscription;
 use sp_core::{
@@ -88,11 +89,11 @@ pub use crate::{
         RawEvent,
     },
     extrinsic::{
+        derive_mortal_period,
         PairSigner,
         SignedExtra,
         Signer,
         UncheckedExtrinsic,
-        DEFAULT_MORTAL_PERIOD,
     },
     frame::*,
     metadata::{
@@ -179,17 +180,17 @@ impl<T: Runtime> ClientBuilder<T> {
             rpc.system_properties(),
         )
         .await;
+        let metadata = metadata?;
+        let mortal_period = derive_mortal_period(&metadata).ok();
         Ok(Client {
             rpc,
             genesis_hash: genesis_hash?,
-            metadata: metadata?,
+            metadata,
             properties: properties.unwrap_or_else(|_| Default::default()),
             runtime_version: runtime_version?,
             _marker: PhantomData,
             page_size: self.page_size.unwrap_or(10),
-            signed_options: ClientSignedOptions {
-                mortal_period: Some(DEFAULT_MORTAL_PERIOD),
-            },
+            signed_options: ClientSignedOptions { mortal_period },
         })
     }
 }