A new theoretical formula, used for the determination of the chemical composition distribution of copolymers, is proposed in this paper. The composition distribution of styrene in chlorinated butyl rubber/polystyrene comb graft copolymer was obtained by gel permeation chromatography utilising a combination of refractive index and ultraviolet detection. It is a more convenient and time-saving method for the characterization of the molecular structure parameters of a copolymer compared to the conventional experimental method.