The vaporization of the samples of the compositions Ga2O3 + LaGaO3, LaGaO3 + La4Ga2O9, and La4Ga2O9 + La2O3 was investigated using Knudsen effusion mass spectrometry in the temperature range 1494-1937 K. The partial pressures of the gaseous species O-2, Ga, GaO, Ga2O, and LaO were determined over the samples investigated. The equilibrium partial pressures were used for the calculation of the thermodynamic activities of the components at 1700 K. Gibbs energies of formation of LaGaO3(S) and La4Ga2O9(S) at 1700 K from the component oxides were derived from the thermodynamic activities as -46.4 +/- 4.7 and -99.2 +/- 7.9 kJ.mol(-1), respectively. The results were compared with the literature data obtained using other methods.