NO reduction by CH 4 over a 40% La 2O 3/ γ-Al 2O 3 catalyst in the absence and presence of O 2 in the feed was studied. The addition of either CO 2 or H 2O to the feed produced a reversible inhibitory effect on the rate similar to that observed with unsupported La 2O 3; however, the extent of rate inhibition was considerably smaller than on unsupported La 2O 3. At 973 K, either CO 2 (9%) or H 2O (2%) in the feed decreased activity by about 35% in the absence of O 2 and by only 20% with excess O 2 in the feed. In the absence of O 2, a reaction mechanism previously proposed for La 2O 3 was altered to include competitive CO 2 and H 2O adsorption and to give the following rate expression for N 2 formation: r N 2 = k′P NO P CH 4 (1+K NO P NO +K CH 4 P CH 4 +K CO 2 P CO 2 +K H 2 O P H 2 O ) 2 . This equation fit the data well, had apparent activation energies of 14–25 kcal/mol, and gave thermodynamically consistent enthalpies and entropies of adsorption. Stable rates at 973 K with O 2 and either CO 2 or H 2O in the feed were between 0.94 and 0.99 μmol N 2/s/g catalyst. In the presence of excess O 2, after CO 2 and H 2O adsorption were again included, a rate equation proposed earlier for La 2O 3 again provided a good fit to the data with H 2O in the feed as well as thermodynamically consistent parameters determined under integral reaction operation. However, with both CO 2 and excess O 2 in the feed, this rate expression could not provide thermodynamically meaningful parameters from the fitting constants even though it fit the data well. This was attributed to a major contribution from the alumina to the overall rate, because CO 2 had no significant effect on NO reduction on alumina, but it inhibited this reaction on La 2O 3. A reaction model was proposed for γ-Al 2O 3 that gave the rate expression for total CH 4 disappearance due to both combustion and NO reduction over γ-Al 2O 3 (r CH 4 ) T = k′ com P CH 4 P O 2 0.5+k′ NO P NO P CH 4 P O 2 0.5 (1+K′ NO 2 P NO P O 2 0.5+K CH 4 P CH 4 +K O 2 0.5P O 2 0.5+K CO 2 P CO 2 +K H 2 O P H 2 O ) 2 , which gave a satisfactory fit to the data along with thermodynamically consistent parameters. The second term in this equation, which represents the rate of N 2 formation, was then combined with the rate equation for N 2 formation on pure La 2O 3 in the presence of O 2 to describe overall catalyst performance, and the data were fit well, assuming that La 2O 3 composed 6.8% of the total surface area, a value close to that of 6.1% obtained from XRD line-broadening calculations.