Abstract Diode-array detection has been applied to evaluate the chemical composition of block copolymers from chromatographic data. Two different modes of chromatography have been used for separation: size exclusion chromatography and adsorption chromatography, based on tertiary mixed eluents, comprising tetrahydrofuran, acetonitrile and dichloromethane on a microcolumn, packed with 5 μm wide-pore silica gel (mean pore size 300 Å). U.v. spectral data acquired on-line were manipulated to generate spectral suppression chromatograms in order to selectively detect each component of a block copolymer chromatographed under both size exclusion and critical thermodynamic conditions. The content of polystyrene and poly(methyl methacrylate) residues in a series of model block copolymers of nominally constant composition (∼ 1:1, w/w), but differing in molecular weight, could be rapidly determined by this novel combination of chromatographic selectivity enhanced by digital discrimination. Thus, diode-array detection has significant potential for fast on-line monitoring of polymer processing.