diff --git a/src/spaces-inl.h b/src/spaces-inl.h
index 2f01164f89..8b2eab0c45 100644
--- a/src/spaces-inl.h
+++ b/src/spaces-inl.h
@@ -93,17 +93,21 @@ Address Page::AllocationTop() {
 
 
 void Page::ClearRSet() {
-#ifndef V8_HOST_ARCH_64_BIT
   // This method can be called in all rset states.
   memset(RSetStart(), 0, kRSetEndOffset - kRSetStartOffset);
-#endif
 }
 
 
-// Give an address a (32-bits):
+// Given a 32-bit address, separate its bits into:
 // | page address | words (6) | bit offset (5) | pointer alignment (2) |
-// The rset address is computed as:
+// The address of the rset word containing the bit for this word is computed as:
 //    page_address + words * 4
+// For a 64-bit address, if it is:
+// | page address | quadwords(5) | bit offset(5) | pointer alignment (3) |
+// The address of the rset word containing the bit for this word is computed as:
+//    page_address + quadwords * 4 + kRSetOffset.
+// The rset is accessed as 32-bit words, and bit offsets in a 32-bit word,
+// even on the X64 architecture.
 
 Address Page::ComputeRSetBitPosition(Address address, int offset,
                                      uint32_t* bitmask) {
@@ -115,7 +119,7 @@ Address Page::ComputeRSetBitPosition(Address address, int offset,
   *bitmask = 1 << (bit_offset % kBitsPerInt);
 
   Address rset_address =
-      page->address() + (bit_offset / kBitsPerInt) * kIntSize;
+      page->address() + kRSetOffset + (bit_offset / kBitsPerInt) * kIntSize;
   // The remembered set address is either in the normal remembered set range
   // of a page or else we have a large object page.
   ASSERT((page->RSetStart() <= rset_address && rset_address < page->RSetEnd())
@@ -131,8 +135,10 @@ Address Page::ComputeRSetBitPosition(Address address, int offset,
     // of the object:
     //   (rset_address - page->ObjectAreaStart()).
     // Ie, we can just add the object size.
+    // In the X64 architecture, the remembered set ends before the object start,
+    // so we need to add an additional offset, from rset end to object start
     ASSERT(HeapObject::FromAddress(address)->IsFixedArray());
-    rset_address +=
+    rset_address += kObjectStartOffset - kRSetEndOffset +
         FixedArray::SizeFor(Memory::int_at(page->ObjectAreaStart()
                                            + Array::kLengthOffset));
   }
@@ -160,14 +166,9 @@ void Page::UnsetRSet(Address address, int offset) {
 
 
 bool Page::IsRSetSet(Address address, int offset) {
-#ifdef V8_HOST_ARCH_64_BIT
-  // TODO(X64): Reenable when RSet works.
-  return true;
-#else  // V8_HOST_ARCH_64_BIT
   uint32_t bitmask = 0;
   Address rset_address = ComputeRSetBitPosition(address, offset, &bitmask);
   return (Memory::uint32_at(rset_address) & bitmask) != 0;
-#endif  // V8_HOST_ARCH_64_BIT
 }
 
 
diff --git a/src/spaces.h b/src/spaces.h
index 8ce807fcd4..299a7644d2 100644
--- a/src/spaces.h
+++ b/src/spaces.h
@@ -93,13 +93,14 @@ class AllocationInfo;
 // bytes are used as remembered set, and the rest of the page is the object
 // area.
 //
-// Pointers are aligned to the pointer size (4 bytes), only 1 bit is needed
+// Pointers are aligned to the pointer size (4), only 1 bit is needed
 // for a pointer in the remembered set. Given an address, its remembered set
 // bit position (offset from the start of the page) is calculated by dividing
 // its page offset by 32. Therefore, the object area in a page starts at the
 // 256th byte (8K/32). Bytes 0 to 255 do not need the remembered set, so that
 // the first two words (64 bits) in a page can be used for other purposes.
 // TODO(X64): This description only represents the 32-bit layout.
+// On the 64-bit platform, we add an offset to the start of the remembered set.
 //
 // The mark-compact collector transforms a map pointer into a page index and a
 // page offset. The map space can have up to 1024 pages, and 8M bytes (1024 *
@@ -217,16 +218,24 @@ class Page {
   // Page size mask.
   static const intptr_t kPageAlignmentMask = (1 << kPageSizeBits) - 1;
 
+  // The offset of the remembered set in a page, in addition to the empty words
+  // formed as the remembered bits of the remembered set itself.
+#ifdef V8_TARGET_ARCH_X64
+  static const int kRSetOffset = 4 * kPointerSize;  // Room for four pointers.
+#else
+  static const int kRSetOffset = 0;
+#endif
   // The end offset of the remembered set in a page
   // (heaps are aligned to pointer size).
-  static const int kRSetEndOffset= kPageSize / kBitsPerPointer;
-
-  // The start offset of the remembered set in a page.
-  static const int kRSetStartOffset = kRSetEndOffset / kBitsPerPointer;
+  static const int kRSetEndOffset = kRSetOffset + kPageSize / kBitsPerPointer;
 
   // The start offset of the object area in a page.
   static const int kObjectStartOffset = kRSetEndOffset;
 
+  // The start offset of the remembered set in a page.
+  static const int kRSetStartOffset = kRSetOffset +
+      kObjectStartOffset / kBitsPerPointer;
+
   // Object area size in bytes.
   static const int kObjectAreaSize = kPageSize - kObjectStartOffset;