A series of sodium borosilicate glasses were synthesized with KReO 4 or Re 2 O 7 , to 10,000 ppm (1 mass%) target Re, to assess effects of large concentrations of rhenium on glass structure and fto estimate solubility of 99 Tc, a radioactive component in low active waste nuclear glasses. Rhenium was used as a surrogate for 99 Tc for laboratory testing, due to similarities in chemistry, ionic size, and redox. Magic angle spinning nuclear magnetic resonance, Fourier transform infrared spectroscopy, and Raman spectroscopy were performed to characterize the glasses. Si was coordinated in Q 2 and Q 3 units, Al was four-coor-dinated, and B was mostly three-coordinated. The rhenium additions did not have significant effects on the glass structure up to approximately 3000 ppm Re by mass, the maximum concentration that remained dissolved in glass. Rhenium likely exists in isolated ReO À 4 anions in the interstices of the glass network, as evidenced by polarized Raman spectrum of the Re glass in the absence of sulfate. Analogous to SO 2À 4 in similar glasses, ReO À 4 is a network modifier and above solubility forms alkali salt phases on the surface and in the bulk. Comparisons of phase separation and crystallization in MoO 2À 4 containing borosili-cate glasses can also be made to ReO À 4 containing glasses.