0.8[xB(2)O(3)-(1 - x)P2O5]-0.2Na(2)O (with 0 <= x <= 1) glasses have been characterized by solution calorimetry at 298 K in acid solvent. The experimental data showed a strong negative departure of the enthalpy of mixing from the ideality described by the equation (in kJ/mol): Delta H=x(1 - x)(-660.2 + 570x). The results were interpreted on the basis of the structural data. Enthalpies of mixing were consistent with sub-regular solution behaviour. (C) 2005 Elsevier B.V. All rights reserved.