The paper presents the results of calculating the change in the standard Gibbs energies in the reactions of calcium oxide with silicon dioxide in the solid state when recording the composition of oxides and compounds per mol of oxygen atoms in the 298,15 ³ T ³ 1800 K temperature range. The maximum change in the standard Gibbs energy was observed for the reaction of formation of Ca0,500(Si1,2)0,500O, which is the most durable compound in the CaO – Si1,2O system (m.p. 2403 K), from oxides. The minimum temperature of the stable chemical compound Ca0,600(Si1,2)0,400O based on the calculated thermodynamic data is 1,610 K.