Osmotic coefficients of the solutions of ampicillin sodium and penicillin sodium in water at T = 298.15 K were measured using the isopiestic technique. The experimental osmotic coefficients have been correlated using the ion interaction model of Pitzer, e-NRTL model of Chen, NRF and a fourth-order polynomial in terms of molality. The vapor pressures of the solutions and the solvent activities have been calculated from the osmotic coefficients. Reliability of the models in expression of the osmotic coefficients were compared on the basis of standard deviation of the fittings. (c) 2012 Elsevier B.V. All rights reserved.