Reactions between nitrogen gas and metal borides and between graphite and metal borides have been studied to obtain qualitative information on the relative stabilities of the metal boride phases. At high temperatures many borides are inert toward carbon or nitrogen. The most stable borides are found to be those of Ti, Zr, Nb, and Ta of the fourth and fifth groups of the periodic table. Data are summarized in terms of limits to the heats of formation.