Dynamic effects brought about by the introduction of small amounts of CO2 and N2O in the reaction feed during the oxidative dehydrogenation of propane to propene on alpha-NiMoO4 catalysts are reported. CO2 promotes oxidation probably via the formation of adsorbed oxygen species, O(a), formed by the dissociation of CO2 on NiMoO4. Catalysts in presence of CO? work in a high oxidation state increasing conversion. CO2 is not an inert non-selective product. It can play an active role during the reaction. It could, via O(a): (i) oxidise propane to propene, (ii) increase the oxidation state on the surface of the catalyst and (iii) in high concentration, promote the non-selectivity of the catalyst. In presence of N2O, catalytic sites work in a more reduced state. N2O improves selectivity, probably by: (i) inhibiting the adsorption of O-2 and/or (ii) increasing reduction rate of the catalysts. The O* produced by dissociation of N2O can oxidise propane to propene. Adjusting the concentration of promoters could be an useful tool to modulate selectivity during the catalytic reaction. (C) 2003 Elsevier Science B.V. All rights reserved.