Hydrogen/deuterium exchange has been determined for mixtures of n-octane-d18/methylcyclohexane-do and n-octane-d0/methylcyclohexane-d14 as well as n-octane-d0/methylcyclohexane-d0/D2 at low (300-400-degrees-C) and high (482-degrees-C) temperatures. H/D exchange occurred in both reactants and products at the low temperatures. At 482-degrees-C, exchange did not occur in the reactants but did occur in the products. The results are consistent with the high temperature conversions occurring by a mechanism with a rate-limiting step of adsorption of the reactant.