Size-resolved aerosol mass and gas concentrations were measured during the European Arctic Aerosol Study (EAAS). A thermodynamic equilibrium model was applied to the data in order to (1) test whether gas-aerosol equilibrium appeared to be obtained by NH3, HNO3, HCl, HCOOH, and CH3COOH, and (2) test the sensitivity of gas-aerosol equilibrium to several nonvolatile organic acids that were measured. Model results indicated that all submicrometer, accumulation-mode aerosols appeared to be near equilibrium with NH3. Supermicrometer, coarse-mode aerosols of recent marine origin appeared to be out of equilibrium with HNO3 and closer to equilibrium with HCl, while continentally influenced aerosols sometimes appeared to be near equilibrium with both HNO3 and HCl. However, the observed gas-aerosol partitioning of HCOOH and CH3COOH could not be explained by effective Henry's law partitioning, consistent with other studies. Nonvolatile organic acids measured were methanesulfonate, oxalate, succinate, and glutarate. Sensitivity tests indicated that methanesulfonate retained ~30% of NH4+ under marine conditions but had <3% impact on other species and under other conditions. Whereas oxalic acid was predicted to be ~15--30% dissociated in the aerosol solution, succinic and glutaric acids were predicted to be <10% dissociated, limiting their ability to influence gas-aerosol partitioning. Together, the three dicarboxylic acids were responsible for retaining 0--2% of predicted NH4+ and displacing 0--6% of predicted Cl- and NO3-. Model results were sensitive to the assumed mixing state of the aerosols, as well as the degree of aerosol size resolution represented by the model. ¿ 2000 American Geophysical Union