In this paper, the osmotic coefficient, phi, of electrolyte solutions is considered. According to the Gibbs-Duhem equation, the calculation of phi follows from that of the mean activity coefficient, gamma, based on a pseudolattice approach recently proposed. For any given electrolyte, the whole range of concentrations providing gamma <= 1 is considered. The major feature of the pseudolattice approach is given by the fact that y can be calculated without using adjustable parameters where the (upper) concentration, c(lim), exists at,which the electrolyte solution exhibits gamma = 1. In the remaining cases, a unique parameter is required, that is, the value of clim that should ideally give gamma = 1 for the electrolyte. Known values of c(lim) from 1 up to 9 M (about) are available for several aqueous electrolytes at 25 degrees C. All formulas in this paper are applied for 1:1, 2:2, 1:2, and 2:1 aqueous electrolytes at 25 degrees C.