A self‐consistent local composition theory for the excess Gibbs free energy of a multicomponent electrolyte solution is developed. The theory incorporates correct pair counting, assumes that like‐charged ions cannot be nearest neighbors of each other, and accounts for the effect of second nearest neighbor interactions. A perturbation analysis of the model, linear in the interaction parameters, is also carried out. For a given solvent and a given class of electrolyte the perturbed theory yields universal functions of molality, which can be weighted by suitable parameter values and added to give the desired property. The parameters can be expressed in a temperature‐independent form. This theory correlates very well the data on osmotic and activity coefficients in single‐solute aqueous solutions up to very high concentrations (up to 20 M) as well as over a range of temperatures. Copyright © 1990 American Institute of Chemical Engineers