The deuterium isotope effect in the steady state CO oxidation rate over Au/gamma-Al2O3 in the presence of H-2 or H2O and the effect of pretreatment on an uncalcined catalyst were studied. In a reaction feed containing 1% CO, 0.5% O-2, and 40.5% H-2 at room temperature, CO oxidation exhibited a deuterium isotope effect (k(H)/k(D)) of 1.4 +/- 0.2. The rate of D-2 oxidation was also slower than the oxidation of H-2, such that the selectivity for CO oxidation was 86% in the presence of D-2 versus 77% in the presence of H-2. In contrast, there was no deuterium isotope effect in a feed containing 1% CO, 0.5% O-2, and 1.5% H2OH2 was also more effective in regenerating a CO oxidation reaction deactivated catalyst than D-2, whereas H2O and D2O were equally effective. The difference was attributed to the different mechanisms with which H-2 or H2O prevented deactivation of the catalyst during CO oxidation. An uncalcined Au/gamma-Al2O3 was rather inactive. It could be activated by treatment with a mixture of H-2 and H2O at 100degreesC, although treatment by either H-2 or H2O alone was ineffective. The observations are consistent with the model of the active site consisting of an ensemble of metallic Au atoms and a cationic Att with,a hydroxyl group.