During the geological disposal of high level waste, the nuclear glass is expected to be first hydrated in water vapour prior to liquid alteration. In this work the vapour hydration of the International Simple Glass (ISG) at 175 deg C and different relative humidities (60%, 80% and 98%), was investigated. The glass hydration was investigated by nuclear reaction analysis *NRA) and Fourier transform infra-red spectroscopy. The chemical and mineralogical compositions of the alteration products were studied using SEM-EDS and u-Raman spectroscopy, respectively. The NRA results gave water diffusion coefficients of 2.31-7.34 x 10-21 m2/s, in good agreement with the literature data on borosilicate glasses altered in aqueous media. The glass hydration increased with relative humidity percentage and the SEM-EDS analysis showed a slight enrichment in Si and loss of Na in the hydrated glass layer compared with the pristine glass. The hydration rate of the ISG glass was little higher than that of the French SON68 glass hydrated using water vapour. The corrosion products were analcime, tobermorite, and calcite, which were typical of the SON68 glass hydrated in similar conditions.
