A method is presented for measuring the composition and molecular weight of block copolymer simultaneously by size exclusion chromatography with a single detector. The method requires intrinsic viscosity and size exclusion chromatographic data on the whole sample and that the detector be responsive only to one of the two monomer units of the block polymer. The response constant and the molecular weight separation curve are first calibrated with homo-polymer standards. The chromatogram of the whole sample is resolved into two peaks cor-responding to homopolymer and block copolymer by peak resolution technique. The composi-tion and the molecular weight of the copolymer are calculated with the aid of the response constant and universal calibration principle. The utility of the method is illustrated using a series of styrene-dimethylsiloxane block copolymer samples on a conventional SEC unit with a differential refractometer as detector.