Abstract To elucidate the tritium release mechanism from the surface of solid breeding materials, surface -OH and -OD on Li 2O were studied at high temperature up to 400°C and under controlled H 2O (D 2O) vapor pressure or H 2(D 2) partial pressure by using a diffuse reflectance method of FTIR (Fourier transform infrared absorption spectroscopy). During the drying process of Li 2O, a large peak of precipitated LiOH was observed at 3662 cm −1. When D 2O vapor was added to a concentration higher than the precipitation limit, the O-D peak from LiOD was observed at 2696 cm −1. Below the limit D 2O vapor pressure, absorption peaks of isolated -OD on the surface were found at least at two positions: 2748 cm −1 and 2717 cm −1. This experimental fact shows that the surface is not homogeneous for D 2O adsorption. The peaks shifted to the lower wave numbers with the precipitation of LiOD at lower temperature. When D 2 was added to He + D 2O sweep gas, the intensity of the peak from isolated -OD became higher, indicating D 2 is dissociatively adsorbed on the Li 2O surface to produce -OD.