In the present study we use the hygrometric method for the determination of relative humidities of aqueous electrolyte solutions. The relative humidities of KCl(aq), Na2SO4(aq), and NaNO3(aq) have been measured for different molalities and at temperatures from (278.15 to 323.15) K. The data on the relative humidities allow the deduction of the water activities and the osmotic coefficients at the temperature of 298.15 K. The solute activity coefficients are also determined for the three salts using Fitter's model. The results obtained by this method are compared with those found in the literature.