Reactivities of hydrogen atom/ions with different carbon and boron nitride (BN) phases were studied by means of frontier orbital theory based on ab initio Hartree-Fock calculations. The results show that there is a significant difference in the reactivity of atomic hydrogen with the sp(2) and the sp(3) carbon phases such that phase selectivity is facilitated during chemical vapor deposition of diamond. In contrast, the reactivities of atomic hydrogen with the sp(2) BN phase and the sp(3) BN phase are similar, indicating the difficulty in obtaining a pure cubic boron nitride phase via chemical vapor deposition. In addition, hydrogen ions show higher reactivity than their neutrals, whereas hydrogen anions show similar reactivity with the two carbon or the two BN phases. CH3 species were found to be promoters for the preferential etching selectivity of hydrogen in BN growth.