The heat of formation of crystalline decaborane has been determined by calorimetric measurement of the heat of decomposition: B10H14(c)→10B(am)+7H2(g),ΔH(25°C)=82.8±5.9kj/mole,(19.8±1.4kcal/mole). With the heat of transition of crystalline to amorphous boron taken as 0.40 kcal/mole, we obtain for the heat of formation of decaborane: ΔHf298.15°=-66.1kj/mole,(-15.8kcal/mole)..