The contributions of lattice oxygen and adsorbed oxygen to the oxidation of butane by VPO catalysts were quantitatively determined by measurement of the reaction kinetics with a novel oscillating microbalance reactor. At 674 K, with the gas composition of butane/oxygen/Ar/He = 1.75/2.1/1.6/balance (vol%), and a gas hourly space velocity (GHSV) of 30 000 h(-1), oxidation of butane by lattice oxygen accounts for only about 5% of the butane oxidation rate, with the remainder carried out by adsorbed oxygen, most likely O-. Thus under steady state reaction conditions, the traditional Mars-van Krevelen mechanism is a minor contributor to the rate.