The measurement of the total vapour pressure of binary, quasi-binary and ternary systems gives us information for the determination of the existence of phases, the existence range of phases and their coexistence and solubility ranges by the investigation of the barograms of the systems, too. By determination of the existence fields we obtain the decomposition mechanism and the dominant gaseous species and from the temperature function of the equilibrium constant we calculate the reaction enthalpy and reaction entropy and from these the enthalpy of formation and the standard entropy. The possibilities of the method are demonstrated on the ternary system Bi/Se/O.