Abstract: XPS has been used to study the thermal stability of the surface oxides on transition-metal foils including Ni, Fe, Cu, Ti, Zr, Nb and Ta formed by oxida- tion of the metals in dry air at different temperatures. The results show that the room-temperature surface oxides (NiO, Fe_2O_3, CuO or Cu_2O, TiO_2, ZrO_2, Nb_2O_5 and Ta_4O_5) are converted to the metals after heating the samples in vacuum at 300~400 ℃. The surface oxides (NiO, Fe_2O_3, Cu_2O, TiO_2, ZrO_2, Nb_2O_5 and Ta_2O_5) formed at 250~400 ℃, however, cn not be converted to the metals by heating the samples in vacuum at the same temperatures. The reduction of room-temperature surface NiO is caused by reaction with surface hydrocarbon contaminants. For room-temperature surface oxides TiO_2, ZrO_2, Nb_2O_5 and Ta_2O_5, the apparent deoxidation may be explained by diffusion of oxygen atoms from the surface into the metallic lattice, the reduction caused by reaction with carbon-containing contaminants, however, can not be completely ruled out because carbides are formed after heating these samples in vacuum. It has been suggested that the lower thermal stability for room-temperature surface oxides can be attributed to its “two-dimensional structure” feature. Since the surface energy is higher and there are more structural defects as well as chemical defects in two-dimensional film, the net results are a weakening of the metal-oxygen bond and a lowering of thermal stability. The higher thermal stability for the high-temperature surface oxides can be explained by surface sintering and the formation of bulk oxide. SEM observation revealed that the room-temperature surface oxide for vapor-deposited Ni film agglomerates into a great number of grains after heating the film at 250 ℃ for 2 hours. Along with surface sintering, reconstruction and phase change of the surface oxides may also occur and thus decrease the surface energy and increase the thermal stability.