Thermochemical analyses are performed for the Ti–O–C, Fe–O–C, Al–O–C, Si–O–C, Mg–O–C, Be–O–C, Y–O–C, Cr–O–C, and U–O–C systems at 1800 K. The role of hydrogen, water vapor, and nitrogen in connection with these systems is exemplified. LogpMexOyvs logpO2and logpCvs logpO2diagrams are prepared to illustrate in a simple manner the relations between condensed phases and the equilibrium pressures of gaseous species over the different condensed phases. The diagrams are used to evaluate the correctness or applicability of available thermochemical data, and to predict or interpret reactions in or between the different systems, with particular reference to vacuum metallurgical processes.