The hard magnetic characteristies of compounds having composition Sm(Co1−y Cu y)z and lying within the phase space 5⋅0?z?8⋅5, 0?y?1⋅0, are strongly influenced by the nature of their metallurgical microstructure. However, by measurement of the anisotropy field, domain size, and angular dependence of the coercivity, and in conjunction with data obtained from transmission electron microscopy, as well as the implications of a theoretical model for the coercivity in these materials, and simple arguments concerning the diluting effect of copper upon the primary magnetic properties of the constituent phases, the principal features of this group of materials may be clarified.