Nickel and aged stainless steel that was composed of nanometer-scale Cr-rich and Fe-rich regions were chosen as model systems for the modification of hydrogen-trap sites. Nickel and stainless steel were annealed and oxidized to modify the types and numbers of hydrogen-trap sites. Deuterium was used in this experiment to enhance the detectability of hydrogen isotope detected in the atom probe analysis. Strongly bound deuterium trap sites are observed and have a higher density than the solubility in lattice. The amount of deuterium trapped in nickel decreases after annealing. This is a result of the modification of trap sites by annealing. Deuterium trapped in the oxide layer and in the oxide-nickel interface of nickel was observed at a sufficient resolution to determine the extent of trapping on an atomic scale, and thus to gather quantitative data on the modification of trap sites by oxidation. The deuterium atoms are trapped into the Cr-rich region on the terrific phase of duplex stainless steel. The deuterium atoms trapped are associated with nickel atoms and oxygen atoms.