Polyacrylamide hydrogels containing bis-[4-{dimethylamino}phenyl] (4-vinyl-phenyl) methyl leucohydroxide (TPMLH) have been studied as ion selective membranes. An understanding of the factors influencing the ionic diffusion rates based on photochromism is presented. The observed increase in methyl orange (anionic) flux after UV irradiation was mainly due to the photoinduced generation of fixed cationic charges in the membrane as opposed to a combined effect of fixed charge generation and membrane structural changes. Methyl orange fluxes in a fully activated membrane (in pH 4) were close to an order of magnitude higher than UV activated membranes. The flux of 4-dimethylamino pyridine (a neutral species) in both systems was essentially unchanged by irradiation. Modeling of the predicted fluxes based on the Nernst-Planck and Donnan equilibrium equations using no adjustable parameters gave good correlation with the observed results.