A photocatalytic reaction mechanism that involves multielectron reduction of molecular oxygen (O-2), as a key step, in photocatalytic decomposition of acetic acid by pristine bismuth tungstate (Bi2WO6; BWO) particles is proposed on the basis of results of studies on (i) comparison of photocatalytic activity trends of BWO, tungsten (VI) oxide and titanium(IV) oxide photocatalyst powders with or without platinum deposits in three photocatalytic reaction systems (oxidative decomposition of acetic acid in aerobic aqueous suspensions, methanol dehydrogenation in deaerated aqueous suspensions and O-2 liberation from deaerated silver salt solutions) and (ii) effects of morphology and structural properties of BWO particles on photocatalytic activities.