diff --git a/core/sr-primitives/src/generic/mod.rs b/core/sr-primitives/src/generic/mod.rs
index 6732722c4db0a..2f944be29dc39 100644
--- a/core/sr-primitives/src/generic/mod.rs
+++ b/core/sr-primitives/src/generic/mod.rs
@@ -35,3 +35,28 @@ pub use self::checked_extrinsic::CheckedExtrinsic;
 pub use self::header::Header;
 pub use self::block::{Block, SignedBlock, BlockId};
 pub use self::digest::{Digest, DigestItem, DigestItemRef};
+
+use codec::Encode;
+use rstd::prelude::*;
+
+fn encode_with_vec_prefix<T: Encode, F: Fn(&mut Vec<u8>)>(encoder: F) -> Vec<u8> {
+	let size = ::rstd::mem::size_of::<T>();
+	let reserve = match size {
+		0...0b00111111 => 1,
+		0...0b00111111_11111111 => 2,
+		_ => 4,
+	};
+	let mut v = Vec::with_capacity(reserve + size);
+	v.resize(reserve, 0);
+	encoder(&mut v);
+
+	// need to prefix with the total length to ensure it's binary comptible with
+	// Vec<u8>.
+	let mut length: Vec<()> = Vec::new();
+	length.resize(v.len() - reserve, ());
+	length.using_encoded(|s| {
+		v.splice(0..reserve, s.iter().cloned());
+	});
+
+	v
+}
diff --git a/core/sr-primitives/src/generic/unchecked_extrinsic.rs b/core/sr-primitives/src/generic/unchecked_extrinsic.rs
index 4f3a32b34d156..b5cd01c2cd9e3 100644
--- a/core/sr-primitives/src/generic/unchecked_extrinsic.rs
+++ b/core/sr-primitives/src/generic/unchecked_extrinsic.rs
@@ -103,9 +103,9 @@ where
 	fn decode<I: Input>(input: &mut I) -> Option<Self> {
 		// This is a little more complicated than usual since the binary format must be compatible
 		// with substrate's generic `Vec<u8>` type. Basically this just means accepting that there
-		// will be a prefix of u32, which has the total number of bytes following (we don't need
+		// will be a prefix of vector length (we don't need
 		// to use this).
-		let _length_do_not_remove_me_see_above: u32 = Decode::decode(input)?;
+		let _length_do_not_remove_me_see_above: Vec<()> = Decode::decode(input)?;
 
 		Some(UncheckedExtrinsic {
 			signature: Decode::decode(input)?,
@@ -123,19 +123,10 @@ where
 	Call: Encode,
 {
 	fn encode(&self) -> Vec<u8> {
-		let mut v = Vec::new();
-
-		// need to prefix with the total length as u32 to ensure it's binary comptible with
-		// Vec<u8>. we'll make room for it here, then overwrite once we know the length.
-		v.extend(&[0u8; 4]);
-
-		self.signature.encode_to(&mut v);
-		self.function.encode_to(&mut v);
-
-		let length = (v.len() - 4) as u32;
-		length.using_encoded(|s| v[0..4].copy_from_slice(s));
-
-		v
+		super::encode_with_vec_prefix::<Self, _>(|v| {
+			self.signature.encode_to(v);
+			self.function.encode_to(v);
+		})
 	}
 }
 
@@ -150,3 +141,20 @@ impl<Address, Index, Call, Signature> fmt::Debug for UncheckedExtrinsic<Address,
 		write!(f, "UncheckedExtrinsic({:?}, {:?})", self.signature.as_ref().map(|x| (&x.0, &x.2)), self.function)
 	}
 }
+
+#[cfg(test)]
+mod test {
+	use codec::{Decode, Encode};
+	use super::UncheckedExtrinsic;
+
+	#[test]
+	fn encoding_matches_vec() {
+		type Extrinsic = UncheckedExtrinsic<u32, u32, u32, u32>;
+		let ex = Extrinsic::new_unsigned(42);
+		let encoded = ex.encode();
+		let decoded = Extrinsic::decode(&mut encoded.as_slice()).unwrap();
+		assert_eq!(decoded, ex);
+		let as_vec: Vec<u8> = Decode::decode(&mut encoded.as_slice()).unwrap();
+		assert_eq!(as_vec.encode(), encoded);
+	}
+}
diff --git a/core/sr-primitives/src/generic/unchecked_mortal_extrinsic.rs b/core/sr-primitives/src/generic/unchecked_mortal_extrinsic.rs
index c494bc00ff428..793532c0749fe 100644
--- a/core/sr-primitives/src/generic/unchecked_mortal_extrinsic.rs
+++ b/core/sr-primitives/src/generic/unchecked_mortal_extrinsic.rs
@@ -113,9 +113,9 @@ where
 	fn decode<I: Input>(input: &mut I) -> Option<Self> {
 		// This is a little more complicated than usual since the binary format must be compatible
 		// with substrate's generic `Vec<u8>` type. Basically this just means accepting that there
-		// will be a prefix of u32, which has the total number of bytes following (we don't need
+		// will be a prefix of vector length (we don't need
 		// to use this).
-		let _length_do_not_remove_me_see_above: u32 = Decode::decode(input)?;
+		let _length_do_not_remove_me_see_above: Vec<()> = Decode::decode(input)?;
 
 		let version = input.read_byte()?;
 
@@ -141,28 +141,19 @@ where
 	Call: Encode,
 {
 	fn encode(&self) -> Vec<u8> {
-		let mut v = Vec::new();
-
-		// need to prefix with the total length as u32 to ensure it's binary comptible with
-		// Vec<u8>. we'll make room for it here, then overwrite once we know the length.
-		v.extend(&[0u8; 4]);
-
-		// 1 byte version id.
-		match self.signature.as_ref() {
-			Some(s) => {
-				v.push(TRANSACTION_VERSION | 0b1000_0000);
-				s.encode_to(&mut v);
-			}
-			None => {
-				v.push(TRANSACTION_VERSION & 0b0111_1111);
+		super::encode_with_vec_prefix::<Self, _>(|v| {
+			// 1 byte version id.
+			match self.signature.as_ref() {
+				Some(s) => {
+					v.push(TRANSACTION_VERSION | 0b1000_0000);
+					s.encode_to(v);
+				}
+				None => {
+					v.push(TRANSACTION_VERSION & 0b0111_1111);
+				}
 			}
-		}
-		self.function.encode_to(&mut v);
-
-		let length = (v.len() - 4) as u32;
-		length.using_encoded(|s| v[0..4].copy_from_slice(s));
-
-		v
+			self.function.encode_to(v);
+		})
 	}
 }
 
@@ -275,4 +266,14 @@ mod tests {
 		assert!(ux.is_signed());
 		assert_eq!(<Ex as Checkable<TestContext>>::check(ux, &TestContext), Err("bad signature in extrinsic"));
 	}
-}
\ No newline at end of file
+
+	#[test]
+	fn encoding_matches_vec() {
+		let ex = Ex::new_unsigned(DUMMY_FUNCTION);
+		let encoded = ex.encode();
+		let decoded = Ex::decode(&mut encoded.as_slice()).unwrap();
+		assert_eq!(decoded, ex);
+		let as_vec: Vec<u8> = Decode::decode(&mut encoded.as_slice()).unwrap();
+		assert_eq!(as_vec.encode(), encoded);
+	}
+}
diff --git a/node/executor/src/lib.rs b/node/executor/src/lib.rs
index 7fc4f51d69255..b0e7ba25f3200 100644
--- a/node/executor/src/lib.rs
+++ b/node/executor/src/lib.rs
@@ -312,7 +312,7 @@ mod tests {
 		construct_block(
 			2,
 			block1(false).1,
-			hex!("e312f8e2111124c57448050aed74c625382ed222f48ab55ada41afacd0a65fa6").into(),
+			hex!("0f107d73773b84409de8a444dd0d39e6d90b18b3ce393b379a4f6bf34226aa0a").into(),
 			None,
 			vec![
 				CheckedExtrinsic {