The adsorption of butene (B-sites), the strong adsorption of butadiene (A-sites) and the sorption of O were studied on (partly) reduced and sintered samples of Bi2MoO6, in connection with studies on the rate of redn. of the catalyst by butene and the rate of reoxidn. of reduced samples. The A-site concn. (A) was independent of the surface area of the catalyst and decreased linearly with the degree of redn. The B-site concn. (B) falls to half its original value if samples are preheated to 600 Deg. B-sites remain present if the catalyst is reduced below 400 Deg but disappear if the redn. is performed above 400 Deg or when a reduced sample is heated above 400 Deg under vacuum. O-sorption is fast on samples reduced below 400 Deg but considerably slower on samples preheated above 400 Deg. For the latter samples a new, weak, mol. type of O adsorption was obsd. The rate of redn. is dependent on (A)(B)2. A mechanism for the catalytic oxidn. of butene to butadiene was proposed in which butene is first adsorbed on a B-site by s-allyl adsorption, the allyl then moving over to an A-site and donating its second H-atom to another B-site. Reoxidn. of a reduced sample starts at the A-site but at higher temps. of redn. an internal rearrangement in the catalyst leads to a transfer of O2- from MoO2 layers to Bi2O2 layers, the A-sites being on the edges of the Bi2O2 layers.