Full Details from Amazon.com : http://www.amazon.com/gp/product/B0091L49VA?tag=onigi-20

4GB Upgrade for a Apple iMac (21.5 and 27-inch Mid 2011) System (DDR3 PC3-10600 NON-ECC )

204-pin SODIMM
DDR3-1333
DDR3 PC3-10600 Unbuffered NON-ECC DDR3-1333 135V 512Meg x 64
The only brand of memory upgrade available directly from a DRAM manufacturer tested to meet the most stringent industry specifications
Install it yourself in about 15 minutes The same top-quality memory that the worlds largest computer manufacturers install in the systems they sell