Intraporous heterogeneous modification of com. nylon (Ny) and polypropylene (PP) microfiltration membranes, both of 0.2 mm pore size, with grafted poly(acrylic acid) (g-PAA) was accomplished by first coating the membranes with a photoinitiator, benzophenone (BP), and then UV irradn. in acrylic acid (AA) solns. in water. The degree of modification (DG) depended strongly on AA concn. (cAA) and UV time. As estd. from PAA homopolymer GPC analyses, av. degrees of graft polymn. between 680 and 2200 were achieved by varying cAA between 10 and 50 g/l. PP-g-PAA and Ny-g-PAA membranes were characterized with SEM, revealing outer and intraporous surface coverage with g-PAA. FTIR-ATR and energy-dispersive X-ray spectroscopy data verified the chem. compn. and a gradient of DG over membrane thickness, but modification also of the bottom layer. Membrane swelling and permeabilities depending on DG and pH - above and below pKgPAA - were studied. PP-g-PAA membranes were almost dimensionally stable, and with intermediate DG (around 1 mg/cm2) the switch height of transmembrane permeability as function of pH was very high (by a factor of 100, even in 100 mM buffer). Ny-g-PAA membranes markedly changed the shape due to modification, swelling and addnl. pH changes; the mech. stability - esp. at DG>1 mg/cm2 - was poor, and the permeability response to pH was less pronounced. Thus, the differences in membrane hydrophilicity caused different surface selectivity, which for Ny was reduced due to sorption of BP and AA during coating and polymn., resp. Two different types of graft-polymer-modified microporous membranes resulted: almost perfect pore filling for PP-g-PAA, but simultaneous matrix and pore modification for Ny-g-PAA.