Copolymer properties depend on several parameters and one of the most important is its composition. Typically, this composition is determined by C-13 nuclear magnetic resonance (C-13 NMR) analysis, which requires a long acquisition time and a fine adjustment of parameters to obtain the desired resolution. In this article, a method to determine the comonomer content of polymers based on the analysis from gel permeation chromatography (GPC) with an infrared (IR) detector is described. This GPC-IR method is faster than NMR and can be carried out routinely during the GPC analysis. Three sets of ethylene/propylene copolymers synthesized using metallocene and Ziegler-Natta catalyst over a wide composition range. Linear and second order polynomial equations were checked for the relation between the ethylene percent and the IR CH3/CH ratio. Average deviation between GPC-IR and NMR composition values was about 1% and allow conclude that this technique can be considered to be a good alternative. POLYM. ENG. SCI., 51:317-322, 2011. (C) 2010 Society of Plastics Engineers