diff --git a/serde2/src/json/ser.rs b/serde2/src/json/ser.rs
index 4f6214bd..f4bf2754 100644
--- a/serde2/src/json/ser.rs
+++ b/serde2/src/json/ser.rs
@@ -220,7 +220,7 @@ fn fmt_f64_or_null<W: Writer>(wr: &mut W, value: f64) -> io::IoResult<()> {
 pub fn to_vec<
     T: ser::Serialize<Serializer<io::MemWriter>, io::IoResult<()>>
 >(value: &T) -> io::IoResult<Vec<u8>> {
-    let writer = io::MemWriter::new();
+    let writer = io::MemWriter::with_capacity(1024);
     let mut state = Serializer::new(writer);
     try!(value.serialize(&mut state));
     Ok(state.unwrap().unwrap())