Abstract: In the presence of electron donor, molybdovanadophosphoricacid (H_(3+n)PMo_(12-n) V_nO_(40)) are proved to possess catalytic activities in oxygen activation and benzene hydroxylation. O_2 absorption kinetics and hydroxylation activity are different for the two reducing agents used in this paper—ascorbic acid and sodium sulfite. Based on the results obtained from potentiometric titration, cyclic voltammogram measurement, electrolytic reduction hydroxylation and UV, ESR spectrum, plausible mechanisms of O_2 activation for both reducing systems could be drawn. Being reduced by ascorbic acid, the anion of heteropoly acid might directly coordinate to O_2 by transfering 3 electrons to it. The control step may be the coordination of O_2 to reduced anion. But in sodium sultite medium, the electron donor might directly take part in the process of O_2 activation forming a precursor complex with the anion. The reducing coordination of electron donor with anion could be regarded as the control step in this system.