Heat capacities and densities of aqueous solutions containing mixtures of simple electrolytes, [NaCl(aq) + CaCl{sub 2}(aq)], [NaCl(aq) + MgCl{sub 2}(aq)], and [CaCl{sub 2}(aq) + MgCl{sub 2}(aq)], have been measured from 298.15 to 373.15 K at 0.6 MPa. For each system, values were obtained for full ranges of mixture compositions for total ionic strengths of approximately 3 and 5 mol/kg. Measurements for NaCl(aq) (0.11--5.95 mol/kg), MgCl{sub 2}(aq) (0.44--5.19 mol/kg), and CaCl{sub 2}(aq) (1.46--5.54 mol/kg) are also reported. The measurements were done using a flow differential heat capacity calorimeter in series with a vibrating-tube densimeter. Values for the apparent molar heat capacities and volumes are reported and compared to literature values and to values predicted by both Young`s rule and Pitzer`s theory.