A comparison is conducted between 4 atmospheric equilibrium mod els: GFEMN, ISORROPIA, SCAPE2, and SEQUILIB. While ISORROPIA, SCAPE2, and SEQUILIB simplify the problem at hand in an effort to reduce computational rigor, GFEMN does not employ many of the simplifying assumptions used in previous models, thus allowing it to accurately predict multistage aerosol behavior and deliquescence depression. We examine model performance for representative atmospheric environments over an extended composition, temperature, and RH domain and against observations in Southern California. The predictions of GFEMN, ISORROPIA, SCAPE2, and SEQUILIB are in general agreement, but the latter 3 do not adequately reproduce multistage deliquescence behavior for multi component systems. The most notable differences in model predictions occur for H+ and aerosol water concentrations; discrepancies in predictions of aerosol nitrate and total dry inorganic PM concentrations are not as significant. The models predict different deliquescence relative humidities for multicomponent systems, but for ammonia poor environments, these discrepancies do not introduce differences in total dry inorganic PM predictions. Against measurements taken during the Southern California Air Quality Study (SCAQS), all models qualita tively reproduce but generally underpredict aerosol nitrate concentrations. Finally, based on its overall agreement with GFEMN and its computational efficiency, ISORROPIA appears to be the model of choice for use in large-scale aerosol transport models. In places where crustal material comprises a significant portion of total PM, SCAPE2 is an alternative.