[en] The oxidation of H-451 graphite by steam helium mixtures was investigated. The reaction rate constants in the kinetic Langmuir-Hinshelwood equations were calculated. It was found that an apparent transition in the reaction mechanism occurred at about 300 Pa (3000 μatm) H₂O. This was analyzed by calculating two unique sets of rate constants for two regimes of water vapor pressure. A third, composite set of constants is also submitted which approximately describes the oxidation rate over the entire range of P/sub H₂O/. The composite set is recommended for computer (or other) calculations should the use of two separate sets prove cumbersome. The three sets of reaction rate constants are listed