Hydrogen is a dominant outgassing species from stainless steel vacuum chambers and components in ultra-and extremely high vacuum. Vacuum firing and oxidation are common practice to attain a low out gassing rate. Only a few studies have been taken so far to measure the hydrogen concentration profiles in ultrahigh vacuum materials, stainless steel,@footnote 1@ copper@footnote 2@ and aluminum.@footnote 3@ These studies utilize high energy (MeV order) ion beams to probe for hydrogen. Unfortunately, because of energy spread of ion beam, the depth resolution is limited to be layers than 7.5nm, which is equivalent to the thickness of the surface oxide layers of stainless steel as well as those of aluminum alloys prepared in the controlled atmospheres. Consequently, means of MeV ion beam technique, it is hardly possible to examine the hydrogen concentration depth profile in the surface oxide layers or the oxide-metal interface. Atom probe - field ion microscopy (AP-FIM) is unique among the family of surface analysis techniques in that it examines only the outermost atomic layer of the surface atom-by-atom and depth profiling is possible by means of layer-by-layer evaporation without disturbing the structure underneath. The atom probe has no mass limitations from hydrogen to heavier elements and is equally sensitive to all elements. The present study employs a position sensitive atom probe (PoSAP),@footnote 4@ which is a recent addition to variants of the atom probe and makes three dimensional chemical analysis with single atom sensitivity possible, to examine the hydrogen concentration depth profiles in the surface layers as well as the oxide-metal interface.@footnote 5@ Nickel and aged stainless steel comprising Cr-rich region and Fe-rich region with nanometer size was chosen as a model system for modification of hydrogen trap site. Annealing and oxidation of nickel and stainless steel are carried out as the method to modify the hydrogen trap site. Deuterium was used for this experiment in order to increase the atom probe detection quantity of trapped hydrogen. The number of deuterium trapped in Nickel decreases after annealing, which is a modification of trap site by annealing. Deuterium trapped in the oxide layer and in the oxide-nickel interface has been observed with sufficient resolution to determine the extent of trapping on an atomic scale, which is a modification of trap site by oxidation. No remarkable segregation of deuterium trap site was recognized in the separated ferritic phase of aged duplex stainless steel, because the modulated structure has a match interface. However, after oxidation, the number of trap site decreases and the trap site tends to move to the interface between Cr-rich region and Fe-rich region and the oxide-metal interface. Furthermore it should be noted that clustered trap sites would be observed with the modification treatment of oxidation. @FootnoteText@ @footnote 1@ L. Westerberg and B. Hjorvarsson, Vacuum 47 (1996) 687. @footnote 2@M. W. Ruckman, M. Strongin, W. A. Lanford, and W. C. Turner, J. Vac. Sci. Technol. A13 (1995) 1994. @footnote 3@K. Kanazawa, M. Yanokura, M. Aratani and Akiyama, Vacuum 44 (1993) 7. @footnote 4@A. Cerezo, T. J. Godfrey and G. D. W. Smith, Rev. Sci. Instrum. 59 (1988) 862. @footnote 5@T. Yoshimura and Y. Ishikawa, J. Vac. Technol. A 12(4), (1994) 2544.