One of the methods for calculating the Gibbs free energy of oxide compounds assumes ideal solution conditions [1]. Application of such concept gives relative deviations from 1% to 20%. Due to the fact that oxide compounds are not ideal solutions, the absolute deviations might have values of several hundreds of kJ mol(-1). CRAIG and BARTON improved the ideal mixing model by introducing correction coefficient concerning Delta G(mix)(id), [2]. In this work, the concept of excess Gibbs energy is adopted for calculation of the Gibbs free energy in order to improve calculated Gibbs free energy of oxide compounds. The suggested equation for excess Gibbs free energy calculation is function of mole numbers of oxide subcompounds (CaO, Al2O3 and SiO2), rather then mole fraction. The proposed equation has three parameters. Two of them, alpha and gamma, are temperature dependent. For parameters fitting, the data of the following oxide systems were used: CaO . Al2O3, CaO2. Al2O3, 3CaO . Al2O3, CaO . 6Al(2)O(3),CaO . SiO2, 2CaO . SiO2, 3CaO . SiO2 and 3CaO . 2SiO(2). Application of the suggested method to the chosen systems enabled the estimation of Gibbs free energy values with relative errors less than 1% (absolute errors between 0.025 and 17.6 kJmol(-1)).