Vitrification is a widely accepted method to immobilize hazardous heterogeneous high-level radioactive waste. Along with their favorable mechanical and thermal properties, the capacity of borosilicate glasses to accommodate a wide range of chemical species makes them excellent candidates for nuclear waste forms. However, the presence of high-valent species such as S6+ in the waste, which resist incorporation into the glass, demands improvements in glass composition to prevent the devitrification of sulfates at target waste-loading levels. A phase-separated sulfate layer can be an environmental threat, as it sequesters radioactive species such as 90Sr2+ and 135,137Cs+, and could contaminate ground-water resources during long-term geological disposal. Based on reports of improved sulfate incorporation in phosphate glasses, I have investigated doping borosilicate glasses with phosphate to evaluate its potential to enhance sulfur loading without compromising chemical durability. I have characterized a series of borosilicate glasses with various S6+ and P5+ contents using x-ray diffraction (XRD) and solid-state nuclear magnetic resonance (NMR) spectroscopy to gain insight into the identities of the devitrified products and to better understand the short-range structure of the glasses. 23Na NMR is complementary to XRD for determining the sulfate phases, whereas 31P NMR provides information about phosphate speciation in the glass. 29Si, 11B and 27Al NMR are used to determine the glassy short- range structure and polyhedral connectivity. Together, these results indicate that the preparation of homogeneous sulfur-bearing glasses requires the balance of the high field-strength cations, P5+ and S6+.