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An example of an isothermal thermodynamic phase diagram of a quaternary system composed of a binary alloy and two oxidants has been calculated under appropriate values of the stability of the corresponding compounds, assuming that each metal forms only one compound with each oxidant. Initially, two-dimensional sections of the complete three-dimensional diagram made along planes corresponding to constant values of one of the variables involved were examined in detail. The actual structure of this type of three-dimensional diagram was then discussed by analyzing the shape and the boundaries of the various regions corresponding to the conditions of stability of each phase taken separately or in combination with other phases.