The heat of mixing of HCl and GaCl3 concentrated isomolal aqueous solutions has been measured, in the 0.0005-0.159 molality and 293-333 K temperature range by a flow-mix microcalorimetric method, to determine the thermodynamic properties of the solutions. The results obtained have been successfully explained by Fitter's ion interaction model. Fitter's mixing coefficients have been calculated and used to study the nature and intensity of ion interactions in the mixed solutions. Using Fitter's parameters, the activity coefficients of the mixture components have been calculated.