Transmission electron microscopy has been used to characterize the rhombohedral 5:19 phase (R3¯m) in a stoichiometric (Sm0.75Pr0.25)5Co19alloy. In some regions, strain‐induced contrast was observed in the 5:19 phase. By contrast analysis, this strain contrast was associated with a displacement vector of the type (1)/(3) [011¯0]. In other regions where the strain‐induced contrast was significantly reduced, faults with the crystal structure of hexagonal (Sm,Pr)2Co7(2:7H) phase (P63/mmc) were observed. The formation of the faults is believed to release elastic energy within the 5:19 phase. In the interface regions between the 5:19 phase and the minor component hexagonal 2:7Hphase, it was found by lattice imaging that the 5:19 and 2:7Hplates are superimposed with a commoncdirection. The orientation relationship between the 5:19 and 2:7Hphases is given by [0002]2:7H∥[0003]5:19Rand [112¯0]2:7H∥[112¯0]5:19R.