We have systematically studied the effect of B2O3 substitution by P2O5 on the properties of four composition series of SnO–P2O5–B2O3 glasses showing constant SnO amounts of 50, 60, 67, and 72 mol%. The results show a non-linear evolution of density, glass transition temperature, and refractive index when B2O3 is substituted by P2O5. These properties experience a steep increase followed by a slow increase as the B2O3/P2O5 substitution increases. The structural changes are analyzed using high-field 11B and 31P MAS-NMR spectroscopy. We show that the properties evolutions due to the mixed anion effect are broken up by the direct binding of three- and four-coordinated borate units to the four-coordinated phosphate unit.