Abstract Ferromagnetic C15-type compounds SmCo x are found for 1.74 ⩽ x ⩽ 2.04. The values of the magnetizations vary from 1.21 μ B (SmCo 1.74) −1 to 1.34 μ B (SmCo 2.04) −1. The Curie temperatures are 230 K, independent of x. The initial hydrogenation of SmCo x leads to a C14-type hydride which contains about four hydrogen atoms per SmCO x . Below 373 K, the C14-type hydride changes into a PuNi 3-type hydride having four hydrogen atoms per SmCo 2. Above 453 K, the C14-type hydride transforms into an amorphous hydride having fewer than three hydrogen atoms per SmCo 2, which in turn decomposes into SmH 2 and cobalt above 573 K. The C14-type and the PuNi 3-type hydrides are considered to be paramagnetic. The amorphous hydride is ferromagnetic up to the decomposition with 2.2 μ B (SmCo 2) −1.