The h eats of fo rmation of dibo ranc and p entabora ne h a ve bee n determined by measurem ents of t he heatH of d eco mpos itio n in to a morphous boron and hy drogen in a calorimeter. The heats of formation at 25° C, from amorphous boron a nd h ydroge n, are 6.73 ± O.52 kcal / mol e for dibora nc (gas) >l nd 12.99 ± O.39 for pe ntabo rane (gas) .