The performances of some cobalt-based catalysts (mixed oxides derived from Co-Al hydrotalcite precursors, cobalt supported on alumina and SBA-15, and a Co-Mn perovskite) in the selective oxidation of toluene to benzaldehyde are analyzed by temperature-programmed reactivity tests. Only one sample (CoOx/CoAl2O3 mixed oxide derived from a hydrotalcite precursor and having a Co:Al ratio of 2) was found active and selective and further examined in flow reactor transient reactivity tests. The latter tests confirmed the results, but indicated a fast deactivation. The analysis of the catalytic performances of this sample in the presence or absence of oxygen and the results of temperature-programmed desorption tests after toluene and oxygen absorption suggest that a different reaction mechanism from the more known Mars-van Krevelen redox mechanism (oxidation by lattice oxygen and reoxidation of the reduced catalyst) could be responsible for the catalytic performances of these catalysts. A series of evidences indicates that tentatively adsorbed oxygen, possibly as peroxo species, are present on the catalysts and responsible for the catalytic behavior. It is also shown that the presence of CO2 in the feed reduces the rate of deactivation, possibly due to the formation of more stable peroxocarbonate species. (c) 2004 Elsevier B.V. All rights reserved.