The present study is an examination of the catalyst deactivation of a silica-supported bismuth-molybdenum complex oxide, and that of catalysts used in the absence of bismuth, for the oxidative dehydrogenation of 1-butene. Due to the detection of deactivation, the molar ratio of 1-butene against oxygen in the reactant gas was adjusted to a ratio similar to that used in industrial processes where reaction temperatures average 100 K higher. Regardless of the presence or absence of bismuth in the catalysts, the conversion of 1-butene was decreased by 6 h on-stream. Both the progress of the coking from the inlet to the outlet of the catalyst and the reduction of molybdenum in the catalysts directly contributed to the deactivation. X-ray photoelectron spectrometry revealed that a greater reduction of molybdenum in the near-surface region and a smaller partial pressure of oxygen (P(O-2)) in the reactant gas, although the molybdenum on the surface was not reduced at all. This indicated that the lattice oxygen was pumped from the near-surface region to the surface during the reaction and the oxygen-poor conditions of the near-surface region both in the gas and catalyst phases were formed at a smaller P(O-2), which resulted in the enhancements of both the reduction of molybdenum and that of coking. Based on the thermogravimetric analysis, the silica-supported bismuth-molybdenum complex oxide used at P(O-2) = 4.1 kPa (color of the catalyst = black) was increased in weight while that used at P(O-2) = 16.4 kPa (color of the catalyst = gray) showed a weight decrease, which indicated that the weight decrease caused by the reduction in molybdenum in the near-surface region used at 4.1 kPa was greater than the weight increase from the coking. It was concluded that the reduction in molybdenum followed by the coking on the catalyst surface were the main factors in the catalyst deactivation.