Sr8/7TiSy (y = 2.84-2.97) has been prepared by controlling the partial pressure of sulfur. The XRD and the ED patterns could not be indexed because of incommensurate structures; they were, therefore, indexed by four-dimensional formalism with simple lattice parameters of A, C-Ti and C-Sr, in a hexagonal setting. However, a commensurate structure was obtained at y = 2.84 in Sr8/7TiSy, the XRD and ED patterns of which were indexed by three- and four-dimensional formalisms. The sulfides, including incommensurate and commensurate structures, could be regarded as a single phase with a homogeneity range of sulfur of y = 2.84-2.97, when the four-dimensional formalism was used. On the other hand, from the viewpoint of three-dimensional formalism, the structure contains two penetrating hexagonal subcells. The lattice parameter "a" in hexagonal setting is common to both subcells, while c is given by c = pC(Ti) = qC(Sr), where p and q are integers, and C-Ti and C-Sr are the periodicities in c-direction of the two subcells, respectively. Hence, there is an infinite number of the structures in the composition range of y = 2.84-2.97 in Sr8/7TiSy. (C) 2002 Published by Elsevier Science Ltd.