The mechanisms and kinetics leading to a loss of molybdenum from bismuth molybdate catalysts with SiO2 and Al2O3 as supports, respectively, were studied as the basis of research into the reasons for the loss of molybdenum from single MoO3 catalysts. It was found that the loss of molybdenum was due to the sublimation of MoO2(OH)(2), formed by the reaction of active species with water vapor. The kinetics leading to the loss of molybdenum in the presence of water vapor are expressed as r = kVC(s)(3.5). The difference in the rate of molybdenum loss from single MoO3 catalysts and bismuth molybdate catalysts are discussed.