Previous tests have shown that glass is highly corroded in the presence of smectite 4a, a clay under consideration for use as a repository engineered barrier material. Alteration experiments with French R7T7 glass were therefore conducted in moist clay media containing 5 wt% of a siliceous additive. Five siliceous materials were tested: silica gel, pozzolana, window glass, R7T7 glass frit and R7T7 glass, all in powder form. Following an initial 28-day preconditioning period during which the clay/additive/leachant system was allowed to reach equilibrium, a glass specimen was placed for 7 or 28 days in the moist clay medium. Glass alteration was characterized by the mass loss and by surface analysis. After 28 days in contact with the silica gel and with the three glass powders the mean glass corrosion rate calculated from the mass loss values was about 0.01 g.m−2d−1, versus 1.4 g·m−2d−1 with clay alone and 0.3 g·m−2d−1 with pozzolana. The use of a suitable additive can thus considerably attenuate the corrosive effects of moist smectite in contact with R7T7 glass.