In this study, the effects of iron and magnesium on International Simple Glass (ISG) alteration were studied throughout 511 days of aqueous leaching experiments. The aim was to determine by thorough characterization, the nature of the alteration products that control glass alteration. Iron and magnesium were added separately or together in solution as FeCl2 and MgCl2 salts, with monthly additions to compensate for their consumption. The alteration degree was determined by leachate analyses (ICP-AES) and the alteration products composition, morphology and microstructure were characterized (SEM, TEM-EDX and XRD). The results indicated that magnesium and iron increase glass alteration, forming tri-octahedral smectites with the same (Fe + Mg)/Si ratio. With iron, two kinds of silicates precipitate with the same composition but with a different morphology, whereas with magnesium alone, a single Mg-silicate forms. Moreover, it was found that the glass alteration rate drops when the pH stabilizes at a minimum value of 7.8 for Mg-silicates and 6.2 for Fe-silicates. At this point the secondary silicates stop precipitating. This result was confirmed by geochemical simulation and the solubility product of these silicates was estimated considering the presence or absence of aluminum in their structure. Finally, a two-step process was proposed to explain the location of the secondary phase precipitation: firstly in solution and at the solution/gel interface, and secondly in a highly porous upper zone of the gel.