The synthesis and microstructural characterization, by means of selected-area electron diffraction and high-resolution electron microscopy, of Ba7Rh6O18 is reported. This material, isostructural to Ba7Co6O18, is formed by rows of one trigonal prism and five face-sharing octahedra running parallel to the c-axis of a trigonal unit cell with parameters a = 1.004(5) and c = 3.165(7) nm. The substitution of Sr by Ba is accommodated by means of twin formation due to a rearrangement of the rows of polyhedra.