Oxygen interaction with Pd(110) has been studied in a wide range of pressures (Po 2 =10 −6 –100Pa) and temperatures (T=400–600K) by X-ray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED) and thermal desorption spectroscopy (TDS). The amount of oxygen absorbed by Pd(110) single crystal grows to more than 100ML when the reaction temperature and the O 2 exposure are increased. Several reconstructed and oxide structures are sequentially formed on Pd(110) as the amount of adsorbed and absorbed oxygen increases. During oxygen adsorption (1×2) reconstruction occurs and a c(2×4) structure is formed on Pd(110) at θ=0.5ML. When the amount of absorbed oxygen increases from 0.5 to 2.0ML, oxygen penetrates into subsurface Pd layers to the depth exceeding 15–20Å while retaining the c(2×4) structure. When the amount of absorbed oxygen is between 2 and 5ML, the surface is subjected to reconstruction with the formation of a surface oxide with θ∼0.8 that has a complex LEED diffraction pattern. Further increase of the amount of absorbed oxygen beyond 5ML leads to oxygen location in the subsurface region forming PdO clusters. They gradually grow to form an almost continuous layer composed of PdO clusters.