Measurements on a series of random copolymers with units of methylmethacrylate, butylacrylate, styrene and maleic-anhydride are performed. A method is used which consists in fractionating the copolymer by size exclusion chromatography (SEC), collecting 30-40 fractions and then recording both the NMR and MALDI spectra of the fractions. In a successive 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. A model for SEC fractionation of copolymers is developed. It allows to predict the composition and D(x) of the SEC fraction (D(x) is the ratio between the number-average and the weight-average molar mass, x is the fraction number). Offline SEC-MALDI measurements are performed, in order to measure experimentally D(x) for the random copolymer. The agreement between theoretical and experimental values for D(x) and for the composition of the SEC fraction (as derived from SEC-NMR measurements) is good. The predictions of the model were also compared with literature data for SEC fractionation of a copolyester with units of butileneadipate and butilenesebacate. The agreement between theoretical and experimental values is more than satisfactory.