In this work, the mean activity coefficient of NiCl2 in the (NiCl2 + NiSO4 + H2O) mixed electrolyte system were determined by using the potentiometric technique at T = 298.15 K. The measurements were carried out on the galvanic cell without liquid junction of the type: Ni-ISE/NiCl2 (m(A)), NiSO4 (m(B)), H2O/Ag-AgCl over total ionic-strengths from (0.0035 to 6.000) mol . kg(-1) for different series of salt molal ratios r (r = m(NiCl2)/m(NiSO4) = 1.00, 2.50, 5.00, 10.00 and 15.00). The PVC based nickel ion selective electrode (Ni-ISE) and Ag-AgCl electrode used in this work were prepared in our laboratory and had a reasonably good Nernst response. The activity coefficients results were interpreted by using the Harried rule and Pitzer model. The Harned coefficients and the Pitzer mixing parameters have been evaluated for the system. The results show that the Harned rule is obeyed under studied system. The Pitzer model is adequate for the description of our aqueous system. Furthermore, the parameters obtained with the Pitzer model were used to calculate the values of the mean activity coefficients of NiSO4, the osmotic coefficients and the excess Gibbs free energies for the whole series of the studied mixed electrolyte systems. (C) 2009 Elsevier Ltd. All rights reserved.