The separation of ethanol from water by pervaporation was carried out using polysiloxaneimide (PSI) membranes. Polyimide/poly(dimethylsiloxane) (PI/PDMS) segmented copolymer with various PDMS contents in the main chain were prepared by polycondensation of oligomers of amine-terminated PDMS with or without 4,4’-oxydianiline (ODA) and 3,3’,4,4’-benzophenonetetracarboxylic dianhydride (BTDA) followed by thermal dehydration. The upper curing temperature of 300-degrees-C was used to insure complete imidization. All the membranes, PSI with and without ODA, had an affinity to water at room temperature. Interestingly, the separation factor toward water decreased to unity with increasing feed temperature up to a specific temperature (T(ci)). Thereafter, the membrane had an affinity to ethanol. The separation factor toward ethanol increased to a maximum when the feed temperature increased to T(max). This phenomenon will be discussed in detail. As a result, we were able to obtain the desired separation factor for water or ethanol via a carefully controlled feed temperature. A separation factor for ethanol of 3.9 and a 325 g/m2 h permeation rate with a 72.5-degrees-C feed temperature was obtained. In addition, a separation factor for water of 7.5 and a 207 g/m2 h permeation rate at a 20-degrees-C feed temperature was also obtained. The PSI membranes containing ODA had larger permeation rates and lower separation factors toward ethanol than those without ODA. The PSI membrane without ODA had a maximum separation factor for ethanol at a feed concentration of 30 wt% ethanol.