Memory Module 8GB 1333MHz DDR3 (PC3-10600) - 2X4GB SO-DIMMs RAM for iMac 21.5" inch 2.5GHz Quad Core Intel Core i5 with Thunderbolt I/O 2011 MC309LL/A (PARTS-QUICK BRAND) Halogen Free; ROHS Speed: PC3 10600 1333MHz Form Factor: 204 pin SODIMM Capacity: 8GB 2X4GB KIT Warranty: Lifetime