The universal quasi-chemical (UNIQUAC) model is widely used to describe nonideal fluid-phase equilibria in membrane processes. One way to overcome the problem of an unknown membrane molar mass, is to compute the membrane process performance in terms of mass fractions. Since molar and mass-based activity coefficients are not the same, the UNIQUAC equation is converted to mass-based expressions for correct simulation of membrane process performance. This conversion is described in view of general application for the description of mass transport through membranes. A validation of the conversion is performed successfully by calculating literature data by both versions of the UNIQUAC model, and comparing the results. Furthermore, potential applications of the theory are shown. Both liquid and vapor sorption isotherms are calculated. The results show good agreement between the theory described in this article and experimental results. (C) 2010 American Institute of Chemical Engineers AIChE J, 57: 1889-1896, 2011