The effect of ion-irradiation on the state of hydrogen in Nb doped with hydrogen of a concentration of ½H=½M ¼ 0:023 (hydrogen-to-metal-atom ratio) from the gas phase has been investigated at room temperature by the channelling method utilizing a nuclear reaction 1 H( 11 B,) with a 11 B beam of an energy of about 2 MeV. It has been demonstrated that, before irradiation, hydrogen is located at a tetrahedral (T) site, while, after irradiation at room temperature with the 11 B beam in the off-channelling direction up to a dose of about 1:4 Â 10 16 /cm 2 , hydrogen is displaced from a T site by 0.45 -0.55 Å towards its nearest neighbour lattice point. It is concluded that this irradiation-induced site change of hydrogen is due to trapping of hydrogen by irradiation-introduced monovacancies. Hydrogen is located at a site displaced from one of the nearest neighbour T sites of a vacancy by 0.45 -0.55 Å towards that vacancy.