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<or =1 is considered. The major feature of the pseudolattice approach is given by the fact that gamma can be calculated without using adjustable parameters where the (upper) concentration, clim, 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 clim 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.