In the present work we have applied Fitter's model for the representation of equilibrium properties of the system H3PO4-H2O at 25 degreesC. The database used consist mainly of experimental values of osmotic coefficients and of speciation data over the apparent molality range 0-9 mol kg(-1) of H3PO4 The model considers four solutes: H3PO4, H+, H2PO4- and H5P2O8-. Seven adjustable parameters are used and several assumptions which allows to lower the number of adjustable interaction parameters. The results from the model compare well with experimental data. The results are also compared with previous results from models used to describe the equilibrium properties of the system H3PO4-H2O. It is shown that, contrary to our approach, these older models do not represent the speciation of the system accurately.