A model that described the thermodynamics of aqueous phosphoric acid solution for the overall phosphoric acid concentration range from dilute to a molality of 24 mol kg(-1) was developed. In this model, two chemical equilibria (H3PO4 --> H+ + H2PO4- and H2PO4- + H3PO4 --> H5P2O8-) were taken into account and an ionic-strength dependence of the second virial coefficient for the interaction between ion and undissociated phosphoric acid molecule was introduced. The model was used to calculate the activities of water, the molalities and activity coefficients of the main species H+, H2PO4- H5P2O8-, and H3PO4 in aqueous phosphoric acid solution. The calculated activities of water in aqueous phosphoric acid solution are in good agreement with those determined experimentally up to overall phosphoric acid molality of 24 mol kg(-1) with a standard deviation of less than 0.01.