A critical evaluation of the mean acivity and osmotic coefficients in aqueous solutions of thirty‐five uni‐bivalent electrolytes at 298.15 K is presented. The systems which have been treated are ammonium orthophosphate, guanadinium carbonate, 1,2‐ethane disulfonic acid,m‐benzene disulfonic acid, ammonium decahydroborate, and the unibivalent compounds of lithium,sodium, potasium, rubidium, and cesium. Osmotic coefficients were calculated from direct vapor pressuremeasurements, from isopiestic measurements and from freezing‐point depression measurements. Activity coefficients were calculated from electromotive force measurements on galvanic cells without transference and from diffusionmeasurements. Given are empirical coefficients for three different correlating equations, obtained by a weighted least squares fit to the experimental data, and tables consisting of the activity coefficients of the compounds, the osmotic coefficients and activity of water, and the excess Gibbs energy of the solution as functions of the molality for each electrolyte system. The literature coverage is through the computerized version of Chemical Abstracts of September 1979.