The understanding of hydrogen bonding in magnesium and magnesium based alloys is an important step toward its prospective use. In the present study, a density functional theory (DFT) based, full-potential augmented plane waves method of calculation, extended with local orbitals (FP-APW+lo), was used to investigate the stability of MgH2 and MgH2:TM (TM = Ti and Co) 10 wt% alloys and the influence of this alloying on hydrogen storage properties of MgH2 compound. Effects of a possible spin polarisation induced in the system by transition metal (TM) ions were considered too. It has been found that TM-H bonding is stronger than the Mg-H bond, but at the same time it weakens other bonds in the second and third coordination around a TM atom, which leads to overall destabilization of the MgH2 compound. Due to a higher number of d-electrons, this effect is more pronounced for Co alloying, where in addition, the spin polarisation has a noticeable and stabilising influence on the compound structure. (C) 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.