Several available models for predicting organic aerosol activity coefficients are compared by examining their effect on predictions of secondary organic aerosol (SOA) concentrations and aerosol water uptake in laboratory and atmospheric systems. Activity coefficient models studied include Ideal behavior, the Wilson and NRTL equations, and UNIFAC in standard form, with revised interaction parameters, and with no temperature dependence. Wilson and NRTL binary interaction parameters were fit from UNIFAC predictions of activity coefficients. SOA model simulations were performed for different combinations of primary organic aerosol (POA) composition, SOA composition, and relative humidity. All of the activity coefficient methods predict similar results for mixtures of similar components, but for highly dissimilar aerosol mixtures, the Ideal, Wilson and NRTL models tend to predict higher SOA concentrations and aerosol water uptake compared to UNIFAC. Different versions of UNIFAC gave nearly identical results for most scenarios. Computational requirements are lowest for Ideal, followed by Wilson, and then NRTL and UNIFAC models. (C) 2004 Elsevier Ltd. All rights reserved.