First-principles calculation reveals that the Ti-H interactions are energetically favorable with negative heats of formation, and H atoms could occupy octahedral and tetrahedral interstitial sites of alpha-Ti simultaneously due to their small energy difference. Calculation also shows that hydrogen concentration plays an important role in determining brittle/ductile behavior of Ti-H phases, and densities of states suggest that a bonding transition from mainly covalent to mainly metallic appear for Ti-H phases when the H/Ti ratio reaches about 1/8. The calculated results agree well with experimental observations and could clarify the controversies of the Ti-H system in the literature. (C) 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.