A comprehensive equation for the thermodynamic properties of the system InCl3-HCl-H2O at 25 degrees C in the ion-interaction (Pitzer) equation form is generated on the basis of a very recent and comprehensive array of electrochemical cell measurements of the HCl activity, together with older published measurements of the activity of InCl3 in mixtures with 0.02 m HCl. Alternate equations with and without explicit consideration of the ion pair InCl2+ as a separate species are tested. Excellent agreement is obtained on either formulation between calculated and measured activities, although considerable uncertainty remains concerning the standard potential for the In electrode. Suggestions are made for additional experiments to reduce this uncertainty.