A theoretical investigation using density functional theory (DFT) has been carried out in order to understand the molecular mechanism of dihydrogen activation by means of transition metal dioxides MO 2 (M = Ti, Zr and Hf), according to the following reaction: MO 2 + H 2 → MO + H 2 O. B3LYP/6-311++G(2df,2pd)/SDD methodology was employed considering two possible reaction pathways. As first step the hydrogen activation by M=O bonds yields to metal-oxo hydride intermediates O=MH(OH). This process is spontaneous for all metal dioxides, and the stability of the O=MH(OH) species depends on the transition metal center. Subsequently, the reaction mechanism splits into two paths; the first one takes place passing through the M(OH) 2 intermediates yielding to products whereas the second one corresponds to the direct formation of the product complex OM(H 2 O). A two state reactivity mechanism was found for TiO 2 system whereas for ZrO 2 and HfO 2 no spin-crossing processes were observed. This is confirmed by CASSCF/CASPT2 calculations for ZrO 2 that lead to the correct ordering of electronic states not found by DFT. The results obtained in the present paper for MO 2 molecules are consistent with the observed reactivity on surfaces.