Novel mechanism of hydrogen interaction with transition metals via stepwise reversible dissociative ionization of H2 molecule is proposed instead of a commonly accepted dissociative adsorption. It involves ionization of H2 to molecular ion (H2+)ad on the outer surface of metal phase, its subsequent absorption and dissociation within the metal phase into (H+)ab ions, i.e., absorbed protons, as described by: H2⇄(H2+)ad+e− and (H2+)ad⇄(H2+)ab⇄2(H+)ab+e−. Absorption here is treated as adsorption on the inner surface of the tetrahedral and octahedral voids within metal lattice. The mechanism is based on the first principles and explains consistently the dependence of mechanical properties of metals on the amount of absorbed hydrogen as well as the mechanism of hydrogenation and hydrogen transport through the metals. The proposed dissociative ionization mechanism is well supported by thermodynamic and steric arguments. In the case of noble metals the presented mechanism carries versatile character as it is valid for both gaseous phase and aqueous solutions.