diff --git a/backwardsCompatibility.md b/backwardsCompatibility.md
index 285b4edb..1e11d4b8 100644
--- a/backwardsCompatibility.md
+++ b/backwardsCompatibility.md
@@ -14,7 +14,9 @@ cost. The fee schedule for eWASM is yet to be specified.
 ## Identification of code
 We assume there is some sort of code handler function that all clients have 
 implemented. The code handler identifies the instruction set type by whether it
-starts with WASM's magic number or not.
+starts with the *eWASM preamble* or not.
+
+The *eWASM preamble* consists of an invalid EVM1 opcode (`0xEF`) followed by the WASM magic number.
 
 The WASM magic number is the following byte sequence: `0x00, 0x61, 0x73, 0x6d`.
 
diff --git a/contract_interface.md b/contract_interface.md
index 06e3e672..573507fa 100644
--- a/contract_interface.md
+++ b/contract_interface.md
@@ -6,6 +6,8 @@ The eWASM Contract Interface (ECI) specifies the structure of a contract module.
 
 Every contract must be stored in the [WebAssembly Binary Encoding](https://github.com/WebAssembly/design/blob/master/BinaryEncoding.md) format (in short, WASM bytecode).
 
+The WASM bytecode must be prefixed with an invalid EVM1 opcode (`0xEF`).
+
 ### Imports
 
 A contract can only import symbols specified in the [Ethereum Environment Interface](./eth_interface.md).