Heat capacities and densities of aqueous solutions containing mixtures of simple electrolytes, [NaCl(aq) + CaCl2(aq)], [NaCl(aq) + MgCl2(aq)], and [CaCl2(aq) + MgCl2(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(-1). Measurements for NaCl(aq) (0.11-5.95 mol.kg(-1)), MgCl2(aq) (0.44-5.19 mol.kg(-1)), and CaCl2(aq) (1.46-5.54 mol.kg(-1)) 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 Fitter’s theory.