Index: src/OFSet.h
==================================================================
--- src/OFSet.h
+++ src/OFSet.h
@@ -219,10 +219,41 @@
  * @param object The object which is checked for being in the set
  * @return A boolean whether the set contains the specified object
  */
 - (bool)containsObject: (nullable ObjectType)object;
 
+/*!
+ * @brief Returns the value for the specified key
+ *
+ * If the key starts with an `@`, the `@` is stripped and
+ * `[super valueForKey:]` is called.
+ * If the key does not start with an `@`, a new set with the value for the
+ * specified key for each object is returned.
+ *
+ * @note Unlike with @ref OFArray, any nil values are removed!
+ *
+ * @param key The key of the value to return
+ * @return The value for the specified key
+ */
+- (nullable id)valueForKey: (OFString*)key;
+
+/*!
+ * @brief Set the value for the specified key
+ *
+ * If the key starts with an `@`, the `@` is stripped and
+ * `[super setValue:forKey:]` is called.
+ * If the key does not start with an `@`, @ref setValue:forKey: is called for
+ * each object.
+ *
+ * @note A @ref OFNull value is translated to nil!
+ *
+ * @param value The value for the specified key
+ * @param key The key of the value to set
+ */
+- (void)setValue: (nullable id)value
+	  forKey: (OFString*)key;
+
 /*!
  * @brief Returns an OFEnumerator to enumerate through all objects of the set.
  *
  * @returns An OFEnumerator to enumerate through all objects of the set
  */

Index: src/OFSet.m
==================================================================
--- src/OFSet.m
+++ src/OFSet.m
@@ -21,10 +21,11 @@
 #import "OFSet.h"
 #import "OFSet_hashtable.h"
 #import "OFString.h"
 #import "OFArray.h"
 #import "OFXMLElement.h"
+#import "OFNull.h"
 
 static struct {
 	Class isa;
 } placeholder;
 
@@ -210,10 +211,62 @@
 
 - (size_t)count
 {
 	OF_UNRECOGNIZED_SELECTOR
 }
+
+- (id)valueForKey: (OFString*)key
+{
+	OFMutableSet *ret;
+
+	if ([key hasPrefix: @"@"]) {
+		void *pool = objc_autoreleasePoolPush();
+		id ret;
+
+		key = [key substringWithRange: of_range(1, [key length] - 1)];
+		ret = [[super valueForKey: key] retain];
+
+		objc_autoreleasePoolPop(pool);
+
+		return [ret autorelease];
+	}
+
+	ret = [OFMutableSet setWithCapacity: [self count]];
+
+	for (id object in self) {
+		id value = [object valueForKey: key];
+
+		if (value != nil)
+			[ret addObject: value];
+	}
+
+	[ret makeImmutable];
+
+	return ret;
+}
+
+- (void)setValue: (id)value
+	  forKey: (OFString*)key
+{
+	if ([key hasPrefix: @"@"]) {
+		void *pool = objc_autoreleasePoolPush();
+
+		key = [key substringWithRange: of_range(1, [key length] - 1)];
+		[super setValue: value
+			 forKey: key];
+
+		objc_autoreleasePoolPop(pool);
+		return;
+	}
+
+	if (value == [OFNull null])
+		value = nil;
+
+	for (id object in self)
+		[object setValue: value
+			  forKey: key];
+}
 
 - (bool)containsObject: (id)object
 {
 	OF_UNRECOGNIZED_SELECTOR
 }

Index: tests/OFSetTests.m
==================================================================
--- tests/OFSetTests.m
+++ tests/OFSetTests.m
@@ -16,10 +16,11 @@
 
 #include "config.h"
 
 #import "OFSet.h"
 #import "OFArray.h"
+#import "OFNumber.h"
 #import "OFAutoreleasePool.h"
 
 #import "OFEnumerationMutationException.h"
 
 #import "TestsAppDelegate.h"
@@ -128,8 +129,16 @@
 		ok = true;
 	}
 
 	TEST(@"Detection of mutation during Fast Enumeration", ok);
 
+	TEST(@"-[valueForKey:]",
+	    [(set1 = [[OFSet setWithObjects: @"a", @"ab", @"abc", @"b", nil]
+	    valueForKey: @"length"]) isEqual: [OFSet setWithObjects:
+	    [OFNumber numberWithSize: 1], [OFNumber numberWithSize: 2],
+	    [OFNumber numberWithSize: 3], nil]] &&
+	    [[set1 valueForKey: @"@count"] isEqual:
+	    [OFNumber numberWithSize: 3]])
+
 	[pool drain];
 }
 @end