Water activities of aqueous electrolyte solutions of HCl(aq), LiCl(aq), NaCl(aq), KCl(aq), CsCl(aq), NH4Cl(aq), MgCl2(aq), CaCl2(aq), and BaCl2(aq) have been determined at T = 298.15 K by the hygrometric method, and at molalities ranging from 0.2 mol (.) kg(-1) saturation. From measurements of droplets diameters of reference NaCl(aq) or LiCl(aq), the dependence of relative humidity on solute concentration was determined. The data on the relative humidities allow the deduction of water activities and the osmotic coefficients at different molalities. Osmotic coefficient data have been described by the ion interaction model of Pitzer. The ion interaction parameters were also determined for each of the studied salts. With these parameters, the solute activity coefficients can be predicted. Our present results have been compared with reported thermodynamic data.