In situ and/or operando DRIFT spectroscopy was applied to the study of the behaviour of a Pd (2 wt.%)/gamma-Al2O3 catalyst, used in methane combustion, under various conditions. Under conventional O-2-rich reaction conditions, spectator species (carbonates and/or formates) could be observed, but no surface intermediate species. No differences were observed under reaction conditions in function of the initial oxidation state of palladium. Formates/carbonates species were identified at low temperature (< 400 degrees C) and disappeared when CO, production started. They are not involved in the overall reaction pathway of methane oxidation and are thus suggested to be spectator species. Their production requires a continuous source of CO and would come from the interaction of CO with OH-groups of the gamma-alumina support. Under O-2-lean (O-2/ CH4 = 2) conditions, CO bridging on metallic palladium (Pd-x(0)-CO) is observed at high temperature (> 400 degrees C), evidencing incomplete oxidation of methane. Based on previous operando Raman studies, a mechanistic scheme is proposed, which takes into account the various surface phenomena observed. (c) 2005 Elsevier B.V. All rights reserved.