For the separation of liquid mixtures. IPN structure could be used to enhance the mechanical strength and the selectivity of membrane by restricting the swelling of the transporting polymer component networks. In this study, for the separation of water-ethanol mixture. composite membranes having polyacrylic acid-poly(butyl methacrylate-co-methyl methacrylate) interpenetrating polymer network (PAA-P(BMA-co-MMA) IPN) skin layer supported on crosslinked and porous poly(BMA-co-MMA) were prepared, The porous x-P(BMA-co-MMA) sublayer was formed by polymerizing monomer mixture in the presence of solvent and followed by immersion precipitation. Morphology changes and the factors involved in the pore formation of the crosslinked porous sublayers were investigated, Final morphology of the sublayer revealed three types of morphology depending on the amount of added solvent and degree of crosslinking: (1) non-porous structure with higher degree of crosslinking; (2) porous cellular structure with lower degree of crosslinking and (3) finger-like structure with lower degree of crosslinking and higher solvent content. We could obtain crosslinked highly porous interconnected poly(BMA-co-MMA) sulayer by polymerizing monomer mixture with 70% NMP and (M) over bar (C) = 40,000. It was found that crosslinked porous structure could be obtained when phase separation occurred before gelation. The thickness of the hydrophilic (PAA)-hydrophobic (P(BMA-co-MMA)) IPN skin layer of the composite membrane was about 10 mum. For the azeotropic water-ethanol mixture (95 wt.% ethanol), the PAA(2)-P(BMA15-co-MMA15)(40) IPN composite membrane had a separation factor of H 4 and a permeation rate of 160 g/m(2) h at 25 degreesC.