Porous silica-zirconia (ZrO2: 50 mol%) membranes of pore size around 0.5 nm were prepared by the sol-gel methods for separation of IPA/water mixtures by pervaporation. The prepared membranes showed a long time dependency in separation performance for about 30 hours before their steady states were attained. The IPA flux decreased gradually down to less than 1 mol.m(-2)h(-1), while the water flux was nearly constant (about 300 mol.m(-2)h(-1)) at 73 mol% of IPA (10 wt% of water) in the feed mixture at 75degreesC, resulting in a drastic increase of separation factor from around 40 to 2500. The membranes also showed a high stability even at high water concentration (60 mol% of water) at 75degreesC, where the water flux exceeded 800 mol.m(-2)h(-1). The time dependency of the separation performance is probably due to the formation of -OH groups on the pore surface and the succeeding adsorption and/or the reaction of IPA molecules which can reduce the effective pore diameter.