diff --git a/src/DeferredCancellation.php b/src/DeferredCancellation.php
index 97b05be..6915f80 100644
--- a/src/DeferredCancellation.php
+++ b/src/DeferredCancellation.php
@@ -37,8 +37,8 @@ namespace Amp;
  */
 final class DeferredCancellation
 {
-    private Internal\Cancellable $source;
-    private Cancellation $cancellation;
+    private readonly Internal\Cancellable $source;
+    private readonly Cancellation $cancellation;
 
     public function __construct()
     {
diff --git a/src/DeferredFuture.php b/src/DeferredFuture.php
index 9f85e95..583653e 100644
--- a/src/DeferredFuture.php
+++ b/src/DeferredFuture.php
@@ -8,10 +8,10 @@ namespace Amp;
 final class DeferredFuture
 {
     /** @var Internal\FutureState<T> */
-    private Internal\FutureState $state;
+    private readonly Internal\FutureState $state;
 
     /** @var Future<T> */
-    private Future $future;
+    private readonly Future $future;
 
     public function __construct()
     {
diff --git a/src/Future.php b/src/Future.php
index bde5ae9..52b9c03 100644
--- a/src/Future.php
+++ b/src/Future.php
@@ -86,7 +86,7 @@ final class Future
     }
 
     /** @var FutureState<T> */
-    private FutureState $state;
+    private readonly FutureState $state;
 
     /**
      * @param FutureState<T> $state
diff --git a/src/Internal/FutureIterator.php b/src/Internal/FutureIterator.php
index 2d46d64..14996d9 100644
--- a/src/Internal/FutureIterator.php
+++ b/src/Internal/FutureIterator.php
@@ -18,11 +18,11 @@ final class FutureIterator
     /**
      * @var FutureIteratorQueue<Tk, Tv>
      */
-    private FutureIteratorQueue $queue;
+    private readonly FutureIteratorQueue $queue;
 
-    private Cancellation $cancellation;
+    private readonly Cancellation $cancellation;
 
-    private string $cancellationId;
+    private readonly string $cancellationId;
 
     /**
      * @var Future<null>|Future<never>|null
diff --git a/src/Internal/WrappedCancellation.php b/src/Internal/WrappedCancellation.php
index 4c5f703..f6a575e 100644
--- a/src/Internal/WrappedCancellation.php
+++ b/src/Internal/WrappedCancellation.php
@@ -10,7 +10,7 @@ use Amp\Cancellation;
 final class WrappedCancellation implements Cancellation
 {
     public function __construct(
-        private Cancellation $cancellation
+        private readonly Cancellation $cancellation
     ) {
     }
 
diff --git a/src/TimeoutCancellation.php b/src/TimeoutCancellation.php
index dbd3fa5..2eff9c0 100644
--- a/src/TimeoutCancellation.php
+++ b/src/TimeoutCancellation.php
@@ -9,9 +9,9 @@ use Revolt\EventLoop;
  */
 final class TimeoutCancellation implements Cancellation
 {
-    private string $watcher;
+    private readonly string $watcher;
 
-    private Cancellation $cancellation;
+    private readonly Cancellation $cancellation;
 
     /**
      * @param float  $timeout Seconds until cancellation is requested.