The hydrogenation of GdNi, TbNi, DyNi and SmNi at room temperature and hydrogen pressure below 1 atm has been studied. Ternary hydrides GdNiH3.2, TbNiH3.4, DyNiH3.4 and SmNiH3.7 crystallize in orthorhombic CrB-type structure (S.G. Cmcm). The hydrides formation is accompanied with the strong lattice expansion. On the basis of neutron diffraction patterns refinement the atomic positions of D atoms and interatomic distances were determined for TbNiD3.4 and DyNiD3.4 samples. Deuterium atoms occupy three types of interstitial sites in the structure of TbNiD3.4 and DyNiD3.4: 4(c)-tetrahedron sites [R3Ni], 8(f)-trigonal bipyramide sites [R3Ni2] and linear 4(b)-sites [Ni-2]. (C) 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.