The relative partial molar enthalpies, Delta(H) over bar SiO2, of SiO2 in SiO2-M2O (M= Li, Na, K and Cs) binary and SiO2-CaO-Al2O3 ternary melts were directly measured by drop-solution calorimetry at 1465 K and 1663 K. Delta(H) over bar SiO2 changes from exothermic to endothermic as silica content increases, confirming the tendency toward immisciblity seen from activity measurements. It is concluded that Delta(H) over bar SiO2 is negative due to acid-base reactions and charge-coupled substitutions when the melt is composed of fewer Q(4) and more Q(3) and Q(2) species, but positive due to structural strain when the melt is composed of mostly Q(4) species. The Delta(H) over bar sio(2) obtained by calorimetry is a useful measure of basicity, when comparing different alkali and alkaline earth oxides.