Abstract: XPS has been used to study air-formed surface oxides on pure copper and iron at both room-temperature and 300℃ and to study the thermal stability of the surface oxides. The results show that the room-temperature surface oxide (Cu₂O, Fe₂O₃ and cupric compounds) can be reduced into metal after being heated at 300~350℃ in high vacuum. The surface oxide (Cu₂O and Fe₂O₃) formed at 300℃, however, can not be reduced into metal after being heated at the same conditions, though CuO can be reduced to Cu₂O. It has been suggested that the room-temperature oxides in the two dimensional surface can differ from the bulk oxides with respect to thermodynamical properties.