Mechanisms of propylene oxidation and spectral identification of active intermediates over the Bi-molybdate catalyst have been investigated using in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) in combination with probe molecules. The results show that the exact IR position of allyl species is at the wavenumber of 1454 cm(-1), which resolves the long-term disagreement about assignment of allyl species. Mechanisms of propylene oxidation are perfected by proposing the mechanisms of side reactions: firstly, acetone precursor is formed via enolic species by oxidation and desorbed as acetone; carbon dioxide is then transformed via such species as formate. carboxylate, and carbonate on the catalyst surface except for the gas-phase reaction. Additionally, the mechanism of the main reaction was also verified. (C) 2008 Elsevier B.V. All rights reserved.