By use of the linear-combination-of-atomic-orbital (LCAO) method for a cluster model, we study the alloying effect on chemical bonding in TiH2 systems. In order to verify that the cluster model is suitable for describing the crystal behavior, the electronic structures in pure dihydrides, such as ScH2, TiH2, VH2, YH2, ZrH2 and NbH2 are calculated. Good agreement of the variant trend of the metal-hydrogen interaction with that of the enthalpy of formation for each pure dihydride is found, and the ionic and covalent contributions to the metal-hydrogen interaction are evaluated quantitatively as a reference for judging their contributions in the alloyed TiH2. The alloying effect on the ionic interaction of alloyed TiH2 is somewhat weaker than that on the covalent interaction. It is found that hydrogen makes a stronger covalent bond with the weaker hydride forming elements and hydride non-forming elements rather than the stronger hydride forming elements if there exist Ti atoms in the neighborhood in the alloyed TiH2. This trend can be understood by the orbital bonding analysis. The energy interval and orbital overlap contributions to the interaction between metal and hydrogen atoms are discussed qualitatively. © 2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.