The thermodynamic properties of some solid solutions in the CaOZrO 2 system have been investigated by using solid electrolyte galvanic cells of the type: O 2, Pt|CaO, CaF 2|CaF 2 ( tF = 1)| δCaO(1 - δ)ZrO 2, CaF 2|Pt, O 2. The influence of CaF 2 added in electrodes on the thermodynamic equilibrium was investigated. It was shown that the heterogeneous field with cubic solid solution reaches the composition x = 0.17 mole of CaO. The results indicate that addition of ZrO 2 to the saturated solid solution produces a significant decrease in the activity of CaO. Measured data were used to calculate thermodynamic parameters of reactions with saturated solid solutions Ca 0.17 Zr 0.83 O 1.83, ZrO 2, and CaZrO 3. At temperatures below 820°C, saturated solid solutions have a tendency to decompose into CaZrO 3 and ZrO 2. A comparison of the thermodynamic results with available data on phase relationships in the CaOZrO 2 system is presented. High thermodynamic stability of SrZrO 3 and BaZrO 3 is one of the reasons for the absence of cubic solid solutions in the system MeOZrO 2 ( MeSr, Ba).