Porous silica membranes were tested for the separation performance by vapor permeation and pervaporation of acetic acid/water mixtures at normal boiling points. Because of the small membrane thickness of about 0.5-1 mu m the membrane showed a large water flux of about 400 mol(.)m(-2.)h(-1) (7.2 kg(.)m(-2.)h(-1)) with a separation factor of 500 for pervaporation at 73 mol% (90 wt%) of acetic acid, while a smaller flux of about 200 mol(.)m(-2.)h(-1) (3.6 kg(.)m(-2.)h(-1)) with a separation factor of about 200 was observed for vapor permeation at the same feed concentration. The smaller water flux for vapor permeation is because of the condensation of the feed mixture on the membrane surface due to the concentration polarization in the vicinity of the membrane surface, causing a temperature rise at the membrane surface due to the heat of condensation. The temperature rise was observed clearly during the vapor permeation tests, and the heat and mass transfer mechanisms are qualitatively discussed.