Borate melts containing <20 mol% Na 2 O have been studied using high-energy synchrotron X-ray diffraction. Temperature dependencies of the mean B-O bond lengths are shown to vary strongly with soda content, by comparison to previous measurements on liquid B 2 O 3 and Na 2 B 4 O 7 . Whereas in liquid B 2 O 3 linear thermal expansion of the BØ 3 units is observed, with coefficient a BO = 3.7(2) 9 10 À6 K À1 , this expansion is apparently slightly suppressed in melts containing <20 mol% Na 2 O, and is dramatically reversed at the diborate composition. These effects are interpreted in terms of changes in the mean B-O coordination number, where the reaction BØ 4 À + BØ 3 ⇌ BØ 3 + BØ 2 O À shifts to the right with increasing temperature. The empirical bond-valence relationship is used to convert measured bond lengths, r BO , to coordination numbers, n BO , including a correction for the expected thermal expansion. This method is more accurate and precise than direct determination of n BO from peak areas in the radial distribution functions. Gradients of Dn BO / DT = À3.4(3) 9 10 À4 K À1 close to the diborate composition, and Dn BO / DT = À0.3(1) 9 10 À4 K À1 for a 13(3) mol% Na 2 O melt are observed, in reasonable agreement with Raman spectroscopic observations and thermodynamic modeling, with some quantitative differences. These observations go toward explaining isothermal viscosity maxima and changes in fragility across the sodium borate system.