In situ photoemission electron spectroscopy (XPS) is used to elucidate the hydrogen etching mechanism in nitrogen implanted iron alloys. The samples were prepared by bombarding stainless steel with a broad nitrogen ion source in a high vacuum chamber. The photoemission spectra evolution on increasing hydrogen ion current is correlated with the nitrided surface properties. The presence of hydrogen is associated with oxygen removal, augmenting the surface nitrogen concentration. The total active sites at the surface are constant, i.e., oxygen competes with nitrogen sites on the surface. The absorbed oxygen is etched following a linear law on hydrogen ion flux. Simultaneously, the formation of metallic nitrides is enhanced. At the working temperature, the efficiency of the process is determined by a characteristic time that depends on hydrogen retention time, water formation and desorption time. (c) 2005 American Vacuum Society.