X-ray photoelectron spectroscopy (XPS) is useful for in situ analysis of catalytically active surfaces under reaction conditions. We conducted in situ observations of CO oxidation on Pd single-crystal surfaces with different orientations under near ambient pressure (NAP) conditions with XPS. In this short review we compare catalytically active surfaces of Pd with the different orientations. Both surface oxides and atomic oxygen chemisorbed on metallic surfaces can be active for CO oxidation depending on surface geometric structure. We propose similarity between the surface oxides and the chemisorbed oxygen from a viewpoint of availability of active oxygen. Catalytic activity of the metallic surface under NAP condition is also compared with that under ultrahigh vacuum conditions. (C) 2015 Elsevier B.V. All rights reserved.