Intraporous heterogeneous modification of commercial nylon (Ny) and polypropylene (PP) microfiltration membranes, both of 0.2 mu m pore size, with grafted poly(acrylic acid) (g-PAA) was accomplished by first coating the membranes with a photoinitiator, benzophenone (BP), and then UV irradiation in acrylic acid (AA) solutions in water. The degree of modification (DG) depended strongly on AA concentration (c(AA)) and UV time. As estimated from PAA homopolymer GPC analyses, average degrees of graft polymerization between 680 and 2200 were achieved by varying c(AA) between 10 and 50 g/l. PP-g-PAA and Ny-g-PAA membranes were characterized with scanning electron microscopy, revealing outer and intraporous surface coverage with g-PAA. FTIR-ATR and energy-dispersive X-ray spectroscopy data verified the chemical composition 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 pK(gPAA) - were studied. PP-g-PAA membranes were almost dimensionally stable, and with intermediate DG (around 1 mg/cm(2)) 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 additional pH changes; the mechanical stability -especially at DG > 1 mg/cm(2) - was poor, and the permeability response to pH was less pronounced. Thus, the differences in membrane material hydrophilicity caused different modification surface selectivity, which for Ny was reduced due to sorption of BP and AA during coating and polymerization, respectively. 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.