Abstract The effect of some cationic amines (spermine, spermidine, hexamethonium bromide and tetramethylammonium bromide) on the mobility of electroosmotic flow (EOF) in open fused-silica capillaries was studied. The EOF was found to be significantly reduced with spermine and spermidine and to a lesser extent with hexamethonium bromide. Tetramethylammonium bromide was found to be ineffective as a flow modifier. Spermine (10–20 μm M) gave a stable reduced EOF at pH 2.50, while 400–500 μ M was required at pH 7.0. In a silica-packed capillary, higher concentrations of spermine were required to significantly change the EOF (up to 1 m M). The very low adsorption coefficients ( M − cm −1) of spermine [ ϵ 200=250, ϵ 210=35] and spermidine [ ϵ 200=118, ϵ 210=3] at low UV wavelengths makes them useful as usrface-charge modification agents when using UV detection, and their low conductivity makes them suitable for use with conductivity detection.