The ionic permeselectivity of a cellulose acetate membrane prepared in our laboratory has been studied from membrane potential and electroosmotic flux measurements. The obtained results show that, at low values of the external salt concentration, the membrane behaves in a cationic-exchange way. The membrane fixed charge concentration has been estimated from the membrane potential measurements as similar to 10(-2) mol/liter. Electroosmotic transport decreases notably when the concentration increases, even reversing its sign at concentrations higher than similar to 10(-1) mol/liter. The weak cationic selective character of the membrane and the fact that the anion mobility is higher than the cation mobility in the membrane phase are both considered responsible for the observed significant sign reversal of the electroosmotic permeability.