Compositional and structural considerations are used to understand viscous behavior of multicomponent borosilicate glasses used in the immobilization of high-level nuclear waste. The presence of alkali and alkaline earth oxides in these systems can be considered as diluents to a highly viscous silica matrix. The extent of dilution is characterized by the presence of singly bonded or nonbridging oxygens. For the analysis, the Arrhenius equation is combined with the number of nonbridging oxygens to develop a predictive temperature-dependent model for the viscosity of these systems based on composition.