The water activities for aqueous solutions of Li2SO4(aq), Na2SO4(aq), K2SO4(aq), (NH4)(2)SO4(aq), and sulphates MgSO4(aq), MnSO4(aq), NiSO4(aq), CuSO4(aq), and ZnSO4(aq) were determined experimentally at a temperature of 298.15 K with a hygrometric method, at molalities in the range from 0.1 mol (.) kg(-1) to saturation. The osmotic coefficients are calculated from these results. The coefficients of Pitzer's model was used to fit the osmotic coefficients for each salt solution. These parameters were used to predict solute activity coefficients for the salts studied. (C) 2003 Elsevier Science Ltd. All rights reserved.