In an extremely high-vacuum (XHV) system, hydrogen outgassing is the most important factor to obtain vacuum pressure below LOX 10(-10) Pa. Titanium is an interesting material for applications in XHV construction. This work will demonstrate a clear correlation between hydrogen outgassing and surface structures of pure titanium in vacuum technology. The titanium surfaces were prepared following treatments such as chemical, electrical, buffing, and mechanochemical polishing. Characterization of oxide layers has been carried out using Auger electron spectroscopy and angle-resolved x-ray photoelectron spectroscopy, and a cross section of the modified layer has been observed by using an optical microscope. The hydrogen distribution in a surface region was measured by elastic recoil detection analysis. The oxide thicknesses were from 20 to 50 nm, although the finished titanium surfaces were covered with TiO2 for chemical polish, electrical polish, buffing polish (BP), and mechanochemical polish (MCP); also, TiO was formed under the top layer for MCP. The thick modified layers (about 1 um) on the surfaces were made in BP and MCP processing, and the modified layers had extremely high-density distributions of hydrogen (H/Ti of about 1 to 2). Thermal desorption spectroscopy measurements indicated that the desorption behavior of hydrogen was not strongly dependent on the thickness of the oxide layer, and the hydrogen outgassing rates from the surfaces of samples were less than 4.0X 10(-7) Pa m s(-1) at room temperature. We report the results of the relation between the quantities of desorbed hydrogen gas and the structure of modified layers on titanium polished surfaces.