The fitting of experimental osmotic and activity coefficient values to Fitter equations requires the use of a large number of adjustable parameters when polar neutral species are present in the system. This is because the Fitter equations consider the lambda(ion-neutral) and lambda(neutral-neutral) as constant interaction parameters for the fitting. The modification of the original Pitzer equations, by considering the dependence of these binary interaction parameters concerning the polar neutral components, on both the ionic strength and the neutral species concentration in the system, leads to a considerable improvement in these fittings. To test the applicability of these modified Fitter equations, they were used to fit several series of data from the literature, related to pure aqueous solutions of different amino acids, or urea, as well as aqueous mixtures of an electrolyte plus an amino acid, or urea.