The thermodynamic properties of NaReO₄, CoCl₂, Co(ReO₄)₂, and HCl have been determined in aqueous solutions up to 300°C by calorimetric methods. It was found that monoatomic cations of the same charge type have the same partial molal heat capacity at high temperatures. Hydrogen ion and oxyanions do not appear to follow this simple behavior. The complex ion formation for FeCl⁺ at 280° was also determined calorimetrically. Comparison of experimental results with aqueous electrolyte theory as developed by Cobble, Sen, and Murray indicated that the modified Born theory can be used successfully to predict Gibbs free energies for aqueous electrolytes up to 300°C. The need for modifications for the more highly-charged ions at the highest temperatures is discussed. A semi-empirical method has been developed to predict partial molar volume of electrolytes at high temperatures.