The true number-average molecular weight, M(n), of copolymers is obtained by GPC coupled with a light-scattering detector even if the composition and therefore the refractive index increment varies with elution volume, provided slices taken are monodisperse with respect to molecular weight and composition. In contrast, only an apparent weight-average molecular weight, M(w)(app), can be obtained by the conventional GPC-light scattering combination, even for a perfect chromatographic resolution. The errors in M(n) associated with nonhomogeneous slices are estimated. Experimental data with mixtures of linear polystyrene and poly(methyl methacrylate) confirm these estimations and indicate that the error in M(n) is less than 10% for the system under investigation.