A modified oxide-reduction-diffusion (ORD) method for the synthesis of single-phase SmCo 5 is described. Samarium oxide, cobalt and calcium (20% excess) are heated in sealed niobium tubes following an optimized temperature programme. The reaction proceeds via Ca 1−x Sm x O as an intermediate and yields SmCo 5 , and CaO, which may be washed off with dilute acetic acid. Single-phase SmCo 5 prepared by this route shows an enhanced reactivity towards hydrogen and reacts already at moderate conditions of p(H 2 )=0.1MPa and T<770K. in situ difference scanning calorimetry and X-ray powder diffraction suggest the formation of ternary hydrides SmCo 5 H x at temperatures around 473 and 573K, and disproportionation into cobalt of poor crystallinity and probably an amorphous binary samarium hydride at 700K, i.e. a hydrogen induced amorphization (HIA) of SmCo 5 . Neutron powder diffraction was carried out on a 154 SmCo 5 sample with the high-intensity neutron powder diffractometer D20 at the Institute Laue-Langevin, Grenoble, France. The CaCu 5 type crystal structure was confirmed (space group P6/mmm, a=500.60(2)pm, c=397.01(2)pm at T=298(1)K) and the magnetic moments along the crystallographic c axis, μ|| c , were refined to be 0.15(6), 2.02(6) and 1.75(5)μ B for Sm, Co1 and Co2 atoms, respectively, in ferromagnetic SmCo 5 . The HIA process was confirmed by in situ neutron powder diffraction at higher pressures of 8.1MPa deuterium gas.