Abstract: Taking the Metal-Organic Vapor Phase Epitaxy (MOVPE) of GaN and (Ga_1-xIn_x)P semiconductors as examples, the pyrolysis effect of NH₃ and PH₃ vapor sources on the composition spaces for growing GaN and(Ga_1-xIn_x)P compounds has been studied. According to the situation of NH₃ and PH₃ pyrolysis in experimental conditions, the thermodynamic models for different pyrolysis status are built. Furthermore, the composition spaces for growing Ⅲ-Ⅴ semiconductors are calculated and predicted with the aid of the Thermocalc software. The correspondence of the theoretical analysis with the experimental data indicates that for the thermodynamic analysis of MOVPE process of GaN and (Ga_1-xIn_x)P semiconductors, the calculation and prediction must be conducted under the conditions of the complete equilibrium or the constraint equilibrium based on the practical pyrolysis of NH₃ and PH₃ vapor sources. The analysis of the complete thermodynamic equilibrium is only applied to some specific temperature region or certain epitaxy process after typical pretreatment of vapor sources.