The electromotive force (EMF) measurements of the cell: Rb-ion selective electrode (ISE)/RbCl (m(A)), Rb2SO4 (m(B)), CH3OH (X), H2O (1-X)/Ag-AgCl with X = 10wt.% on the quaternary system RbCl+Rb2SO4+CH3OH+H2O were carried out at 298.15 K and over total ionic strengths from 0.0050 mol kg(-1) up to near saturated concentration 2.5000 mol kg(-1). The Rb-ISE and Ag-AgCl electrodes used in this work were prepared in our laboratory and exhibited reasonably good Nernst response. The activity coefficients of RbCl in the mixture were directly determined. The experimental data were analyzed by using the Harned rule and Pitzer ion interaction model, and the corresponding model parameters required in the two equations for this system have been evaluated. It was found that the experimental results obey the Harped rule and the Pitzer model can be used to describe the system satisfactorily. The activity coefficients of Rb2SO4, the osmotic coefficients of the mixtures and the excess Gibbs free energy were also calculated. (C) 2004 Published by Elsevier B.V.