The feasibility of determining the compositional heterogeneity of copolymers by thin layer-FID chromatography has been demonstrated. Styrene(ST)-acrylonitrile(AN) statistical copolymer products, which were prepared at different conversion of monomer to polymer, were empolyed as test samples. Good separation was achieved by developing in a concentration gradient, using the system of tetrachloroethan-tetrahydrofuran. By this procedure, the composition distribution curves of ST-AN copolymer products were determined. The result agreed well with what one can expect from the influence of conversion upon the compositional heterogeneity.