In this article, the enrichment factor is shown to be a function of sorption, diffusion, and chemical family. For the separation of fusel oils from ethanol/water mixtures, the enrichment factor of alcohols can be calculated by a new, semi-empirical equation that relates the enrichment factor to the molecular structure of the permeants and the feed conditions. If this relationship can be extended to other solvent systems, it has the potential to be used as a pre-screening tool for the selection of membranes for a given set of feed conditions.