Dehydration of four butanol isomers from their aqueous solutions by pervaporation was studied using a commercial hydrophilic membrane with feed water concentration less 20 wt% at 60 degreesC to 100 degreesC. The effects of feed water content on membrane separation performance were studied in terms of: (1) fluxes and permeances of individual components, and (2) separation factor and selectivity of water to butanol. Efforts were devoted to compare and discuss the advantages and disadvantages of using flux or permeance, and separation factor or selectivity as analysis parameters. Using permeance and selectivity instead of flux and separation factor can significantly decouple the effect of operating conditions on performance evaluation, while clarify and quantify the contribution by the nature of the membrane to separation performance. The effects of temperature on polymeric membranes appear to be significantly different when one plots permeance instead of flux versus feed water content at different temperatures, implying that the traditional flux plots might somewhat mislead the analysis. In addition, analyses indicate that aqueous activity coefficient and saturated vapor pressure appear to play important roles when one evaluates the membrane performance in terms of permeance and selectivity. The effects of temperature and feed water concentration on permeance and selectivity can be elucidated nicely with the aid of the known relationship anion, activity coefficient, saturated vapor pressure, temperature, and feed water concentration. (C) 2004 Elsevier B.V. All rights reserved.