Dissolution of a complex, simulated Magnox Waste (MW) glass in batch dissolution experiments at 60 °C over a period of 56 days is accompanied by extensive development of secondary gels. Gel development has been followed using a range of chemical, spectroscopic and physical means. Initially, a surface layer comprising (hydr)oxides of Fe, Zr and the lanthanides develops at the glass surface. Aluminosilicate gels containing Si, Al, Mg, Sr, Cs and Rb develop in systems where sufficient quantities of glass derived solutes accumulate in the leachate. These gels are hydrous and readily soluble in acidic oxalate solutions. Solution chemistry data is consistent with the development of Cs,Sr-bearing aluminosilicates, silica gel and (hydr)oxides of hydrolysis prone waste components. The experimental results are compared with the predictions of a model that considers kinetically constrained glass dissolution and the precipitation of secondary phases, including a Cs,Sr-bearing aluminosilicate gel.