Heats of mixing of two concentrated isomolal aqueous electrolyte solutions have been measured using a flow-mix microcalorimetric method to determine the thermodynamic properties of the mixed solutions. Systems HCl-KCl and HCl-CaCl2 have been examined in the 0.001-2.0 molality and 293-333 K temperature range. Experimental heat values obtained have been interpreted by Fitter's ion interaction model. Fitter's mixing coefficients have been calculated and used to estimate the nature and intensity of the ion interactions in mixed solutions, as well as the interaction intensity difference caused by different electron configurations of the ions present. Using Fitter's parameters, the activity coefficients of the mixed solutions components have been calculated.