A generally applicable method is used here to measure the weight of copolymer chains which possess a given mass and composition, i.e., to determine the bivariate distribution of chain sizes and composition in copolymers. The method consists of three steps. The copolymer is initially fractionated using size exclusion chromatography (SEC) and fractions are collected. The SEC fractions are analyzed by H-1 NMR to determine both the copolymer abundance and composition, and then by matrix-assisted laser desorption ionization mass spectrometry (MALDI) to determine the molar mass of each fraction. in the third step, bivariate distribution of chain sizes and composition maps are derived from knowledge of the molar mass, weight and composition of the copolymer fractions. The compositional distribution is obtained by summation of the sections of the bivariate distribution which belong to a narrow compositional range. The method was applied to two random copolymer samples containing units of styrene (St) and of maleic anhydride (MAH) reacted up to high conversion. Both samples had a strong compositional drift. At low masses, chains were rich in maleic anhydride, whereas at high masses chains were rich in styrene. The bivariate distribution was a smooth surface with a single maximum fbr both samples. Integration of the surface over narrow compositional ranges yielded the compositional distribution histogram. For one of the two samples, the histogram was relatively smooth and almost flat, whereas for the other sample the histogram was triangle-shaped with a discontinuity in the styrene-rich region.