In this work, the Pitzer ion interaction model and Harned rule were used to illustrate the ternary electrolyte system {CoCl2 + Co(NO3)(2) + H2O}. The activity coefficients were determined using a potentiometric method. The galvanic cell used had no liquid junction of type: Co2+-ISE|CoCl2 (mA), Co(NO3)(2) (m(B)), H2O vertical bar Ag center dot AgCl. The measurements were performed at T = 298.15 K and at total ionic strengths from 0.001 to 6.000 mol . kg (1) for different series of salt ratios r = m(Co(NO3)2)/m(CoCl2) = 1.00, 2.50, 5.00, 7.50, 10.00 and 15.00. The PVC based cobalt ion-selective electrode (Co2+-ISE), the PVC based nitrate ion-selective electrode (NO3 -ISE) and the Ag/AgCl electrode used in this work were prepared in our laboratory and showed a reasonably good Nernst response. The experimental results showed that both Pitzer model and Harned rule were suitable to be used satisfactorily to describe this ternary system. (C) 2012 Published by Elsevier Ltd.