A comprehensive equation for the thermodynamic properties of the systemGaCl3-HCl-H2O at 25°C in the ion-interaction (Pitzer) equation form has been generatedon the basis of a recent and comprehensive array of electrochemical cellmeasurements of the HCl activity at total stoichiometric ionic strengths from 0.01 to 3.0mol-kg−1. Alternate equations with and without explicit consideration of thehydrolyzed product GaOH2+ as a separate species have been tested. Excellentagreement is obtained between the calculated and measured cell potentials forthe formulation, which includes GaOH2+ as an additional species. The effect offurther hydrolysis as well as that of complex formation has been found to benegligible. While a satisfactory set of Pitzer parameters has been found, it wasnot possible to obtain a unique thermodynamic representation for this systembecause of large uncertainties in the first hydrolysis constant of Ga(III) and becauseof redundancies and intrinsic correlations between some of the Pitzer parameters.