Glasses of general formula xSb2O3 (1-x)B 2O3 (0≤x≤0.8) have been prepared by conventional melt- quenching. Mössbauer spectroscopy shows that a fraction of the Sb3+ is converted to Sb5+ and this fraction increases with x. High-field 11B MAS NMR gives well-resolved resonances from boron atoms which are 3- and 4-coordinated to oxygen. The fraction of 4-coordinated boron, N4, goes through a maximum value of 0.12±0.01 at x=0.5. The position of the maximum in N4 is consistent with the cation potential for Sb3+, as observed for other systems. However, the low value of N4 at this maximum is not so readily explained. The values are similar to those predicted if [BO 4]- were stabilised by [SbO4]+ but the trends with composition are different. © 2004 Elsevier Inc. All rights reserved.