The enthalpies of solution of synthetic Mg2SiO4-Fe2SiO4 olivine solid solutions have been measured in Pb2B2O5 melt at 970 K. The heat of solution of forsterite was found to be 15.62 ± 0.3 kcal mol-1 and that of fayalite 9.39 ± 0.14 kcal mol-1. Solid solutions between these end-members exhibit small positive deviations from mixing ideality, asymmetric towards the Fe end-member. In terms of the sub-regular solution model, excess enthalpies of intermediate olivine are adequately represented by the equation Hxs = 2(1000 + 1000XFe) XFeXMg. The enthalpies of solution at 970 K are consistent with high temperature phase equilibrium measurements of activity-composition relationships in the olivine series. Excess entropy terms are not needed to relate the phase equilibrium data to the calorimetric data presented here. The enthalpy of solution of FeSiO3 ferrosilite at 970 K was found to be 4.36 ± 0.10 kcal mol-1. This value, when taken together with calorimetric measurements on fayalite and quartz, is consistent with phase equilibrium investigations of the reaction: 2FeSiO3 = Fe2SiO4 + SiO2 Ferrosilite Fayalite Quartz. These provide a check on the internal consistency of the calorimetric data presented here.