The phase diagrams ErFe 2H x and TbFe 2H x are very similar in the range 0 ≤ x ≤ 4 of absorbed hydrogen. The filling of tetrahedral sites in agreement with an empirical geometrical model gives rise to successive phases as α-solid solution, a cubic hydride, and finally a rhombohedral hydride. In the latter case the distortion must be related to the large magnetostrictive potential of the alloys. The schema of filling the sites by hydrogen has been analyzed using neutron diffraction. It is in perfect agreement with the different behavior of the magnetic species as measured by magnetic experiments, NMR, and 57Fe Mössbauer spectroscopy.