11B and 119Sn nuclear magnetic resonance (NMR) spectroscopies have been used to determine the local environments of boron and tin in binary tin borate glasses of general composition xSnO. (1-x) B 2O3 (0.2≤x≤0.7). The changes in NMR parameters with composition have been correlated with the 'so-called' anomalies in the thermophysical properties of the glasses. The variation in glass density with x shows a change in slope and the glass transition temperature goes through a maximum at x=0.5 which is where B4 (=[BO4]/B total,) also goes through a maximum. This is reminiscent of other binary systems although the deviation from the simple x/(1-x) law occurs earlier than in alkali borate systems; the maximum in B4 is at higher x; and the value of this maximum is 0.32, somewhat lower than usually observed. The 11B MAS NMR spectra have been fitted to three species (B4, B3S and B3A). The maximum in B4 is accompanied by discontinuities in B3A and B3S. The quadrupole parameters for the B3 species also show discontinuities at a value of x=0.5. Static 119Sn NMR spectra were fitted to yield values for the principal components (d11, d22, d33) of the chemical shift tensor. All these components initially increased in a near linear fashion with x but the slope decreased by a factor of ∼4 beyond x=0.5. These observations have been interpreted in terms of change in the type and fractions of species present.