Glasses in the system 40(P 2 O 5 )–x(B 2 O 3 )–(60−x)(Na 2 O) (10⩽x⩽30mol%) have been prepared by the melt-quenching technique. Thermal properties were studied using differential thermal analysis and the relationship between composition and thermal stability was obtained. Structural characterization was achieved by a combination of experimental data (infrared and Raman spectroscopy, 11 B and 31 P solid state NMR). In particular, variations in the phosphate network structure upon addition of B 2 O 3 and Na 2 O were investigated. Analysis of the data indicates that with increasing B 2 O 3 content and decreasing Na 2 O, the glass network shows increasing levels of cross-linking between phosphate and borate units. Evidence of direct B–O–P bonds was observed. In the compositional range investigated, borate groups contain boron almost exclusively in four-fold coordination.