Thermodynamic deuterium substitution effects on reactant conversion and product selectivity were investigated by computer simulation for the stoichiometric partial oxidation of methane. Methane conversion shows a normal deuterium isotope effect, whereas CO selectivity shows an inverse deuterium isotope effect. H-2 selectivity exhibits a large normal deuterium effect. Moreover, deuterium isotope effects are dependent on reaction temperatures and pressures. When reaction pressure is very low, such as 0.001 MPa, all thermodynamic deuterium isotope effects are negligible at 650degreesC or above. Three criteria, based on the deuterium-isotope effects, were proposed to determine whether a partial oxidation reaction of methane is kinetically controlled without accurately measured reaction temperatures.