Waste form glasses that contain substantial quantities of iron, manganese, and aluminum oxides, such as the Savannah River SRL TDS-131 glass, form a thick, hydrated surface layer when placed in contact with water. The dissolution of such a glass has been modeled by Wallace and Wicks, with their “Savannah River Model.” We showed previously that the equations of the Savannah River Model could be fitted to published experimental data if a time-dependent diffusion coefficient was assumed for species diffusing through the surface layer. The Savannah River Model assumes that all of the material dissolved from the glass enters solution, whereas it was observed that substantial quantities of material were retained in the surface layer. An alternative model, presented here, contains a mass balance equation that allows material either to enter solution or to be retained in the surface layer. It is shown that the equations derived using this model can be fitted to the published experimental data assuming a constant diffusion coefficient for species diffusing through the surface layer.