A modified oxide-reduction-diffusion (ORD) method for the synthesis of single-phase SmCo5 is described. Samarium oxide, cobalt and calcium (20% excess) are heated in sealed niobium tubes following an optimized temperature programme. The reaction proceeds via Ca1−xSmxO as an intermediate and yields SmCo5, and CaO, which may be washed off with dilute acetic acid. Single-phase SmCo5 prepared by this route shows an enhanced reactivity towards hydrogen and reacts already at moderate conditions of p(H2)=0.1MPa and T<770K. in situ difference scanning calorimetry and X-ray powder diffraction suggest the formation of ternary hydrides SmCo5Hx 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 SmCo5. Neutron powder diffraction was carried out on a 154SmCo5 sample with the high-intensity neutron powder diffractometer D20 at the Institute Laue-Langevin, Grenoble, France. The CaCu5 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)μB for Sm, Co1 and Co2 atoms, respectively, in ferromagnetic SmCo5. The HIA process was confirmed by in situ neutron powder diffraction at higher pressures of 8.1MPa deuterium gas.