Results obtained by adding gaseous promoters (CO2, N2O and H-2) into the reaction feed are presented for two different reactions: (i) oxidative dehydrogenation of propane (ODP), and (ii) catalytic combustion of methane (CCM). The ODP is performed on a mixture of NiMoO4 and CeO2, by adding 3 vol.% CO2 into the feed, and on a NiMoO4/[Si,V]-MCM-41 mesoporous catalyst, in the presence of 1 or 5 vol.% N2O in the feed. The CCM is carried out (i) on Pd(2 wt.%)/CexZr1-xO2 and Pd(2 wt.%)/gamma-Al2O3 catalysts, on pure CeO2 and on a mixture of Pd(2 wt.%)/gamma-Al2O3 and CeO2 powders, by adding 3 vol.% CO2 into the feed, and (ii) on a Pd(2 wt.%)/gamma-Al2O3 catalyst, in the presence of various amounts of H-2 in the feed. It is shown, through all these various examples, that the activity and/or the selectivity of catalysts can be improved by tuning, in a very controlled manner, the oxidation state of active sites via the use of these gaseous promoters. (c) 2004 Elsevier B.V. All rights reserved.