Separation factors for pervaporation of 5 wt% organic/water mixtures through Ge-ZSM-5 zeolite membranes are strongly correlated with the liquid-phase fugacities of the organic components in the feed. Competitive adsorption coverages probably depend on fugacity. This correlation allows qualitative prediction of zeolite membrane performance and is an important advancement since it is possible to group the separation factors by functionality of the organic component. Therefore, we now have a means to determine the approximate range of separation factors for various mixtures.