In this work, the redox and reaction properties of alpha-Bi2Mo3O12 during isobutene (i-C-4(=)) oxidation are probed by a series of temperature-programmed techniques, including temperature-programmed reduction (i-C-4(=)-TPR), oxidation (TPO), and oxidation reaction (TPOR). The results of i-C-4(=)-TPR suggest the initial reduction of alpha-Bi2Mo3O12 by isobutene with the phase transformation to gamma-Bi2MoO6 and formation of reduced MoO2, and the further deep reduction to form metallic bismuth. On the fully oxidized alpha-Bi2Mo3O12 sample, isobutene oxidation is only able to proceed at high temperature with lattice oxygen as the active oxygen species via Mars-van Krevelen mechanism. On the fresh and pre-reduced catalyst samples, however, the oxygen species formed by the chemisorption of gas-phase oxygen on surface defects and vacancies are also active for isobutene oxidation at low temperature via Langmuir-Hinshelwood mechanism. The combined Langmuir-Hinshelwood and Mars-van-Krevelen mechanism is thus proposed for isobutene oxidation on alpha-Bi2Mo3O12.