diff --git a/source/event-emitter.ts b/source/event-emitter.ts
index da22964..d369166 100644
--- a/source/event-emitter.ts
+++ b/source/event-emitter.ts
@@ -1,6 +1,6 @@
 export type EventMap = Record<string, unknown>;
 
-type Listener<T> = (detail: T) => void;
+type Listener<T> = (detail: Readonly<T>) => void;
 
 /**
  * Internally permits listeners for individual event payloads to be stored
@@ -72,7 +72,7 @@ export class EventEmitter<T extends EventMap> {
      * @returns An off callback that can be called to stop listening for events.
      */
     once<K extends keyof T>(type: K, listener: Listener<T[K]>, debounceMilliseconds?: number): OffCallback {
-        const wrappedListener: Listener<T[K]> = (detail: T[K]) => {
+        const wrappedListener: Listener<T[K]> = (detail: Readonly<T[K]>) => {
             this.off(type, listener);
             listener(detail);
         };
@@ -131,9 +131,12 @@ export class EventEmitter<T extends EventMap> {
         const listeners = this.listeners.get(type);
         if (!listeners) return false;
 
+        // Freeze the payload to make it readonly.
+        const readonlyPayload = Object.freeze(payload);
+
         // Emit the event to all listeners.
         listeners.forEach((entry) => {
-            entry.wrappedListener(payload);
+            entry.wrappedListener(readonlyPayload);
         });
 
         // Return true if there are listeners for the event, false otherwise.
@@ -154,13 +157,13 @@ export class EventEmitter<T extends EventMap> {
      * @param timeoutMs - The timeout in milliseconds.
      * @returns The event payload.
      */
-    async waitFor<K extends keyof T>(type: K, predicate: (payload: T[K]) => boolean, timeoutMs?: number): Promise<T[K]> {
+    async waitFor<K extends keyof T>(type: K, predicate: (payload: Readonly<T[K]>) => boolean, timeoutMs?: number): Promise<Readonly<T[K]>> {
         // Create a promise to wait for the event to be emitted.
         return new Promise((resolve, reject) => {
             let timeoutId: ReturnType<typeof setTimeout> | undefined;
 
             // Create a listener function.
-            const listener = (payload: T[K]): void => {
+            const listener = (payload: Readonly<T[K]>): void => {
                 if (predicate(payload)) {
                     // Clean up
                     this.off(type, listener);
@@ -195,7 +198,7 @@ export class EventEmitter<T extends EventMap> {
         // Create a timeout variable.
         let timeout: ReturnType<typeof setTimeout>;
 
-        return (detail: T[K]) => {
+        return (detail: Readonly<T[K]>) => {
             // If a debounce timer is already pending, clear it before scheduling the next one.
             if (timeout !== undefined) {
                 clearTimeout(timeout);