0.8[xB2O3–(1− x)P2O5]–0.2Na2O (with 0≤ x ≤1) glasses have been characterized by solution calorimetry at 298K 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): ΔH = 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.