The dissolution of International Simple Glass (ISG) was investigated at 90 • C and alkaline conditions with various concentrations of dissolved Si and Ca to unravel the combined effects of those elements on ISG reactivity. Experiments were conducted over durations ranging from 20 days to 3 months. Through morphological, structural, and chemical characterizations, the glass dissolution rate was proven to be strongly correlated with the activity of dissolved silica in the solution. While dissolved calcium did not significantly impact the dissolution rate, precipitation of calcium silicate hydrates (CSH) during the experiments enhanced ISG dissolution rate, though to a modest extent. The 3-months experiments highlighted the strong correlation between the dissolution mechanism and the evolution of the nature of secondary phases in saturated solution. During the first 20 days and at high Si and Ca concentrations, CSH precipitated and aggregated, without preventing the passivating impact of the gel layer at the surface of the glass: the dissolution was controlled by diffusion. Then, a resumption of dissolution occurred between 19 days and 76 days, corresponding to the CSH growth, and a possible mechanistic switch to a hydrolysis-controlled reaction rate. Finally, in some experiments, a drop in pH due to carbonate precipitation was observed along with a decrease in the dissolution rate, falling back in a diffusion-limited regime. Overall, this study shows that at 90 • C, pH = 10 and concentrations of SiO 2 (aq) exceeding 50 % of saturation with respect to amorphous silica, irrespective of Ca concentration but in presence of CO 2 (aq), ISG exhibits a very good chemical durability.