Alkylaminium sulfates are frequently detected in ambient aerosols, and are believed to be important in the nucleation of new particles in the atmosphere, despite their comparatively low gas phase concentrations. In this study water activities and osmotic coefficients have been measured, using a chilled mirror dew point technique, of aqueous mixtures of sulfuric acid and the following alkylaminium sulfates: methylaminium, ethylaminium, dimethylaminium, diethylaminium and trimethylaminium sulfate. The solutions were prepared by mixing solutions of the five corresponding amines and aqueous sulfuric acid and determining the exact aminium to sulfate molar ratios by ion chromatography. The results were correlated using an extended Zdanovskii-Stokes-Robinson equation to enable concentration/water activity relationships to be calculated over the entire composition range from pure aqueous sulfuric acid to pure aqueous aminium sulfates. Comparisons with water activities of ammonium sulfate/sulfuric acid mixtures showed very similar behavior for cation:sulfate ratios of 1:1 (the bisulfate salts) and lower, but that osmotic coefficients for the 2:1 ratio (the aminium sulfates) were much greater than for ammonium sulfate. These results differ from those obtained in another recent study (S. L. Clegg, C. Qiu, and R. Zhang  Atmos. Environ. 73:145–158). The relative values of the osmotic coefficients, in concentrated solutions, suggest that the numbers of methyl or ethyl groups in the aminium ion may have a stronger lowering effect on water activity than the alkyl chain length.