diff --git a/io/src/bridge.rs b/io/src/bridge.rs
index 4079d57fd..fc49a0771 100644
--- a/io/src/bridge.rs
+++ b/io/src/bridge.rs
@@ -1,3 +1,5 @@
+// SPDX-License-Identifier: CC0-1.0
+
 #[cfg(feature = "alloc")]
 use alloc::boxed::Box;
 
diff --git a/io/src/error.rs b/io/src/error.rs
index 172dd3d6a..6ae9719bd 100644
--- a/io/src/error.rs
+++ b/io/src/error.rs
@@ -1,3 +1,5 @@
+// SPDX-License-Identifier: CC0-1.0
+
 #[cfg(all(not(feature = "std"), feature = "alloc"))]
 use alloc::boxed::Box;
 use core::fmt;
diff --git a/io/src/lib.rs b/io/src/lib.rs
index ffb2af0de..f501ad46e 100644
--- a/io/src/lib.rs
+++ b/io/src/lib.rs
@@ -1,3 +1,5 @@
+// SPDX-License-Identifier: CC0-1.0
+
 //! Rust-Bitcoin I/O Library
 //!
 //! The `std::io` module is not exposed in `no-std` Rust so building `no-std` applications which
diff --git a/io/src/macros.rs b/io/src/macros.rs
index a0ed86576..4904a826d 100644
--- a/io/src/macros.rs
+++ b/io/src/macros.rs
@@ -1,3 +1,5 @@
+// SPDX-License-Identifier: CC0-1.0
+
 #[macro_export]
 /// Because we cannot provide a blanket implementation of [`std::io::Write`] for all implementers
 /// of this crate's `io::Write` trait, we provide this macro instead.