In this study the influence of active substances on the rejection of acetic acid was investigated. Eighty-two series of experiments and the results of former studies were used to assign the observed effects to different process parameters (e.g., pressure and temperature) and to the chemical properties of the active substance (e.g., functionality, molar mass, steric aspects). Based on the results, a conceptional modified model of a microporous membrane based on the Preferential Sorption-Capillary Flow Mechanism was developed to describe and explain the effects. By means of this model a prediction of the retention of a substance is possible, based on an analysis of the feed composition solution.