The oxidation of the Pd(111) surface was studied by in situ XPS during heating and cooling in 3×10 −3 mbar O 2 . A number of adsorbed/dissolved oxygen species were identified by in situ XPS, such as the two dimensional surface oxide (Pd 5 O 4 ), the supersaturated O ads layer, dissolved oxygen and the (67×67)R12.2° surface structure.Exposure of the Pd(111) single crystal to 3×10 −3 mbar O 2 at 425K led to formation of the 2D oxide phase, which was in equilibrium with a supersaturated O ads layer. The supersaturated O ads layer was characterized by the O 1s core level peak at 530.37eV. The 2D oxide, Pd 5 O 4 , was characterized by two O 1s components at 528.92eV and 529.52eV and by two oxygen-induced Pd 3d 5/2 components at 335.5eV and 336.24eV. During heating in 3×10 −3 mbar O 2 the supersaturated O ads layer disappeared whereas the fraction of the surface covered with the 2D oxide grew. The surface was completely covered with the 2D oxide between 600K and 655K. Depth profiling by photon energy variation confirmed the surface nature of the 2D oxide. The 2D oxide decomposed completely above 717K. Diffusion of oxygen in the palladium bulk occurred at these temperatures. A substantial oxygen signal assigned to the dissolved species was detected even at 923K. The dissolved oxygen was characterised by the O 1s core level peak at 528.98eV. The “bulk” nature of the dissolved oxygen species was verified by depth profiling.During cooling in 3×10 −3 mbar O 2 , the oxidised Pd 2+ species appeared at 788K whereas the 2D oxide decomposed at 717K during heating. The surface oxidised states exhibited an inverse hysteresis. The oxidised palladium state observed during cooling was assigned to a new oxide phase, probably the (67×67)R12.2° structure.