The copolymer compositions as a function of molecular weight for three poly(styrene-co-vinyl stearate) copolymers of widely varying conversion were determined. A combined gel permeation chromatography–infrared spectroscopy method was used. Theoretical changes in copolymer composition were calculated using reactivity ratios. Comparison of the calculated and observed changes in copolymer composition as a function of molecular weight showed qualitative agreement. However, the observed changes in composition were significantly larger than those calculated.