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A convenient method that enables determination of molecular weight distributions of diblock copolymers with conventional size exclusion chromatography (SEC) without referring to any kind of calibration curve or molar mass-sensitive detecting devices is described. It is based on the well-established procedure to calculate the number-average molecular weight of copolymers from their chemical composition and the molar mass of the first block segment (precursor), applied on an ensemble of monodisperse copolymer fractions. The chemical composition of SEC fractions is calculated from their UV and RI detector traces, and the number-average molecular weight of the precursor, which is to be known from independent measurements, is taken as the molar mass of the first block segment. The molecular weight distributions and averages obtained by this method for selected copolymer samples were found to be in reasonably good agreement with the ones determined by NMR and SEC with on-line viscosity or multiangle laser light scattering detection.