The effect of pH on permeability and the pressure-driven ionic separation of a microporous membrane incorporating a pH-sensitive cross-linked poly(4-vinylpyridine) gel in the pores was examined. Membranes were prepared with cross-linked poly (4-vinylpyridine) constrained in the pores of a poly(ethylene) microporous host membrane. The degree of ionization (protonation) of poly(4-vinylpyridine) in the membranes as a function of pH was determined by potentiometric titration. The pressure-driven flux and salt separation of these membranes were monitored as a function of pH and, consequently, the degree of ionization of the pore-filling polyelectrolyte. The pure water flux was found to decrease reversibly by an order of magnitude when the pH of water was changed from 5.5 to 2.6 by the addition of HCl. Similar reversible flux changes were found with pH-adjusted municipal tap water used as a feed. On the other hand, the rejection of cations in tap water increased sharply with addition of HCl and increase of ionization of the incorporated poly(4-vinylpyridine). The results obtained in this study are explained by microphase transitions taking place in the gel network enmeshed with the porous structure of the support membrane.