16GB Total. FOUR 4GB PC3-10600S 204 pin SODIMM DDR3 1333MHz Genuine CMS brand, CMS is one of the most trusted names in the Computer Memory industry and this product carries a Lifetime Warranty from CMS MAX MEMORY: 32GB SOCKETS 4 COMPATIBLE WITH: Apple iMac "Core i5" 2.7 21.5-Inch (Mid-2011) Mid-2011 - MC812LL/A - iMac12,1 - A1311 - 2428