In order to shed light on the redox behavior of gamma- and alpha-bismuth molybdates (gamma- and alpha-BiMos), the oxide surfaces time-dependently reduced by hydrogen and propene at 723 K and subsequently reoxidized by oxygen at 373-723 K were measured by using XPS. On the oxide surfaces reduced either by hydrogen or by propene, simultaneous generation of Mo5+ and Mo4+ was observed. At the initial stage, the generation of Mo5+ occurred faster than Mo4+. With exposure time, each of the cations increased up to almost equivalent amounts: 34-37 and 7-14% of total Mo atom for the gamma- and alpha-BiMos, respectively, while no reduced bismuth species were found except for the formation of metallic bismuth on the gamma-BiMo reduced by hydrogen for a long time at 723 K. In reoxidation of the reduced surfaces by oxygen, a part of the Mo5+ was oxidized to Mo6+ faster than the Mo4+ below 373 K, while above 473 K, the Mo4+ was oxidized preferentially. The results indicated that the lattice oxide ion bridging Bi3+ and adjacent Mo6+ was more active and mobile than the oxide ion doubly bonded to Mo6+. The surface reduction accompanied the diffusion of lattice oxide ions from the bulk to the surface, while in the reoxidation, the incorporated lattice oxide ions migrated into bulk oxide ion vacancies and to the doubly bonded oxide ion vacancies on the Mo4+ through the bulk.