In this work, the ternary mixed electrolyte system (CoCl2 + CoSO4 + H2O) was investigated based on the Pitzer ion-interaction model by using the potentiometric technique at T = 298.15 K. The mean activity coefficients of CoCl2 in the mixed aqueous electrolyte system were determined on the galvanic cell without liquid junction of the type: Co-ISE vertical bar CoCl2 (m(A)), CoSO4 (m(B)), H2O vertical bar Ag/AgCl over total ionic strengths from (0.001 to 6.000) mol . kg(-1) for different series of salt ratio r (r = m(CoCl2)/m(CoSO4) = 1.00, 2.50, 5.00, 10.00 and pure). The PVC based cobalt ion-selective electrode (Co-ISE) and the Ag/AgCl electrode used in this work were made in our laboratory and had a reasonably good Nernst response. The experimental results obeyed the Harned rule, and the Pitzer model could be used to describe this ternary system satisfactorily. Furthermore, the parameters obtained with the Pitzer model (theta(CISO4) = 0.0719 +/- 0.0099, psi(CoC)l(SO4) = 0.0088 +/- 0.0003) were used to calculate the values of the mean activity coefficients of CoSO4, the osmotic coefficients of water (phi) and the excess Gibbs free energies of solution (G(E)) for the whole series of the studied electrolyte systems. (C) 2010 Elsevier Ltd. All rights reserved.