A study was carried out on propene partial oxidation in the presence and absence of steam over a mixed transition metal oxide. Its objective was to explore mechanisms and key rate controlling steps for this partial oxidation in the presence of added water. The reaction was studied in the concentration ranges (by volume %) C-3(-) less than or equal to 20%; 5% < O-2 < 20%; 0.5% < H2O < 15% with N-2 as a diluent. An Sb/Sn/V oxide catalyst was used. Three techniques were used : (1) transient response of reaction products to a step-change in feed composition, (2) temperature-programmed desorption (TPD) for reactant and product adsorbates, and (3) isotopic transient experiments using O-18(2). TPD and isotopic experiments suggest that water addition disturbs the adsorbate concentrations on the surface through competitive adsorption and creates new sites, leading to a slow increase in the rate of partial oxidation. These experiments also establish that oxygen is not strongly chemisorbed but oxygen exchange with the surface is possible. The existence of at least three groups of adsorption and/or reaction sites was postulated, corresponding to weakly bonded propene at 140 degrees C, strongly bonded propene at 210 degrees C and sites on which propene is converted to oxygenates up to 400 degrees C. only the strongly bonding sites contribute to acid formation, C-C bond scission and total oxidation. With water present, further oxidation of oxygenates is suppressed and carbon dioxide forms only from C-C bond scission. Water may participate in acid formation.