The nonstoichiometry of the equilibrium phases is an important aspect in the study of equilibrium between transition-metals and their oxides. This paper reports our measurement on the deviation from stoichiometry in oxygen-deficient MoO2-x. A diagram showing real oxygen potential isotherms for non-stoichiometric MoO2 and Mo in equilibrium was given. The correction for the standard free energy of formation of MoO2 has been conducted and the Smiltens' intergration used for the pure metal side has been discussed.