The thermodynamic properties, including activity coefficients, osmotic coefficients and excess Gibbs free energy for RbCl and Rb2SO4 aqueous mixtures at T = 298.15 K and in 0.01 mol . kg(-1) to 5 mol . kg(-1) ionic strength, were determined by emf measurements. The Rb-ISE and Ag-AgCl electrodes used in this work were prepared in our laboratory and had a reasonably good Nernst response. The experimental data were fitted by using the Harried rule and Pitzer model. The Harned coefficients and the Pitzer binary and ternary interaction parameters for the system have been evaluated. The experimental results obey the Harned rule. The Pitzer model can be used to describe this aqueous system satisfactorily. (C) 2003 Elsevier Science Ltd. All rights reserved.