Leaching cells were designed to test the alterability of nuclear waste glasses under conditions closely simulating an actual geological repository. This paper summarizes the results of twenty long-duration tests simulating a variety of storage conditions. The effects of the backfilling materials, the canister, glass cracking and crystallization, a activity and the nature of the host rock are discussed. Moreover, an experiment has been in progress for over seven years in a granite medium; after three years the corrosion rate dropped to about 2 × 10−3 g m−2 d−1, and is sustained only by the water renewal due to sampling at regular intervals. These results constitute a valuable data base on R7T7 glass alteration in geological media at 90°C. Glass alteration mechanisms described in pure water appear to be applicable to a hard rock medium. In clay and salt, however, considerable work remains to be done before environmental factors can be taken into account in an overall dissolution model.