Iodine-129 collected from a reprocessing plant is regarded as the dominant nuclide in terms of safety for TRU wastes disposal in Japan. AgI vitrification (AgI-Ag2O-P2O5) is a potential iodine immobilization technique, which has the advantages of less iodine volatilization (low-temperature vitrification) and high volume reduction efficiency (approx. 1/25 the original waste volume). The iodine immobilization property can be evaluated by examining the surface condition of the AgI glass immersed in water. In this study, immersion tests have been performed on AgI glass in pure water in a 3% H2-N2 atmosphere at room temperature, and the surface characteristics have been examined. The thin layer (<4.3 μm) that is formed has been found to consist of AgI, which may act as a barrier, preventing leaching of glass components. The concepts behind the iodine release model have been proposed based on diffusion and the solubility of the components at the glass surfaces.