The adsorption and reactivity of CO and NO in the CO + NO reaction on Pd/Al2O3 and Pd-MoO3/Al2O3 catalysts was studied using DRIFTS, and the chemical state of the catalysts was examined by TRP and UV-DRS while the active sites were probed by TPD and chemisorption of CO and NO. The Pd8Mo catalyst presented a monolayer of Mo on an alumina surface resulting from a polymerization process, identified by DRS measurements. With increasing Mo content MoO3 crystals are formed. Mo was reduced to Mo+4 at lower temperatures in the presence of palladium particles, which favored the formation of bimetallic particles that produce a different kind of site for this reaction. TPD results indicated that the presence of the Mo monolayer favors the formation of N-2 compared to the Pd/Al2O3 catalysts. DRIFTS results indicated NO adsorption on Pd+2, Pd-0, and Mo+delta sites as well as formation of isocyanate on the Pd8Mo catalyst, while CO adsorption occurred on Mo during the reaction at different temperatures. Activity studies showed that the Pd8Mo catalyst is much more active and more selective to N-2 than the monometallic catalyst. A model for the reaction is suggested which shows that Mo+delta is a promoter in the catalytic cycle for NO reduction by CO.