A chemical model was developed for the mixed chloride solution of nickel and cobalt at 25℃. This model consisted of chemical equilibria, mass and charge balance equations. The activity coefficients of solutes and water activity were calculated by using Bromley equation. Cobalt ion has a strong tendency to form complexes with chloride ion such as CoCl₃^(-) and CoCl₄^(2-), while the nickel chloride complex was only NiCI`. Distribution of nickel and cobalt species with the compositions and pH values of the solution was obtained from the analysis of ionic equilibria in mixed chloride solutions. The predicted pH values for NiC1₂-CoC1₂-HCI-H₂O system agreed well with those measured at 25℃ up to ionic strength of 9.53 m.