This paper addresses a hydrogen outgassing mechanism in titanium materials with extremely low outgassing property by investigating the distribution of hydrogen atoms concentration in depth below the surface, and the activation energy for desorption of dissolved hydrogen atoms into the boundary region between the surface oxide layer and the bulk titanium and that of adsorbed hydrogen atoms on the surface. The distribution of hydrogen atoms concentration in depth below the surface was analyzed by a time-of-flight secondary ion mass spectrometry (TOF-SIMS). The activation energy for desorption of dissolved hydrogen atoms was estimated by the thermal desorption spectroscopy (TDS) measurement with various heating rates. The activation energy for desorption of adsorbed hydrogen atoms was estimated by the temperature dependence of the outgassing rate in titanium material. In the titanium material, hydrogen atoms show maximum concentration at the boundary between the surface oxide layer and the bulk titanium. Concentration of hydrogen atoms decreases rapidly at the surface oxide layer, while it decreases slowly in the deep region below the surface layer-bulk boundary by the vacuum evacuation without/with the baking process. The activation energy for desorption of 1.02 eV of dissolved hydrogen atoms into the surface layer-bulk boundary is about three times as large as that of 0.38 eV of the adsorbed hydrogen atoms on the surface. These results suggest that the hydrogen outgassing mechanism in the titanium material is composed the follows processes, i.e. the slow hydrogen atoms diffusion at the surface layer-bulk boundary, quick hydrogen atoms diffusion at the surface oxide layer and rapid desorption of adsorbed hydrogen atoms on the surface. This outgassing mechanism gives very low hydrogen concentration near the surface, which results in the extremely low outgassing rate in titanium materials. (C) 2011 Elsevier B.V. All rights reserved.