The cubic CeNi4Mn compound has been claimed to show a large spin‐polarization, magnetic moment and a high Curie temperature. To verify these properties we have carried out the magnetometric and neutron diffraction experiments on the nominally polycrystalline CeNi4Mn and YNi4Mn compounds. The real stoichiometry was estimated based on X‐ray photoemission and neutron diffraction as CeNi4.2Mn0.8 and Y0.7Ni4.2Mn0.8. The magnetic properties of Y0.7Ni4.2Mn0.8 have been studied with neutron diffraction to investigate the role of the rare earth in CeNi4.2Mn0.8. We have found that the samples crystallize in the hexagonal CaCu5‐type of structure. The deviation from the cubic structure is ascribed to the presence of a small nonstoichiometry. These structural changes are accompanied by a drop of the magnetic parameters. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)