A vanadyl pyrophosphate (VPO) catalyst was investigated for its sequential reduction and oxidation ability for butane oxidation, using a novel oscillating microbalance reactor. Correlation of the redox kinetics with the identities of the vanadium species obtained by potentiometric titration methods suggests that only the near-surface oxygen is readily available for the oxidation of butane; effective redox cycles can only occur between V5+ and V4+; accumulation of V3+ species represents one mode of catalyst deactivation.