Hydrogen is a major residual gas constituent in extremely high vacuum (XHV) and its outgassing behavior has been suggested to be closely related to its concentration and distribution in the surface layers of materials used to fabricate vacuum chambers. This paper presents a preliminary results of a series of study taken to measure the hydrogen concentration in the surface layer as well as its distribution by a three-dimensional atom probe, thus leading to a better under-standing of the outgassing process in XHV. Deuterium gas was adsorbed in a nickel base superalloy single crystal atom probe specimen having γ'/γ interface in the apex region at 773 K in a range of pressure 0.0130.04 Pa for 300600 seconds. Then, atom by atom determination of deuterium distribution and concentration in the surface was performed. The atom probe analysis shows that deuterium is adsorbed only in the outermost surface and no trapping occurs at the γ'/γ interface.