The mixing enthalpies in the 0.8[xB(2)O(3)-(1-x)SiO2]-0.2K(2)O glassy system with 0 <= x <= 1 have been deduced from accurate calorimetric measurements of solution enthalpies in acidic solvent at 298 K. The substitution of SiO2 by B2O3 corresponds to a negative enthalpic effect and suggests the absence of some glassy miscibility gap. The mixing enthalpies are of the same order of magnitude as mixing enthalpies usually found in molten salts. (C) 2006 Published by Elsevier Ltd.