We have correlated simultaneously experimental data for osmotic and activity coefficients of strong electrolyte solutions using the Pitzer equation and a modification of it. The optimal value for the Pitzer b parameter is different from the traditional value of 1.2 when using higher concentrations in its determination. An analysis of the equation shows that it is possible to reduce the model to a three-parameter form that represents the coefficients at molalities as high as 25 with better accuracy than the model as proposed by Pitzer.