Glasses in the system 40(P2O5)-x(B2O3)-(60-x)(Na2O) (10 <= x <= 30 mol%) 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, B-11 and P-31 solid state NMR). In particular, variations in the phosphate network structure upon addition of B2O3 and Na2O were investigated. Analysis of the data indicates that with increasing B2O3 content and decreasing Na2O, 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. (C) 2008 Elsevier B.V. All rights reserved.