First studies of the ternary system SnSe-Bi2Se3 developed in 1960s and 1970s have revealed the existence of a nonstoichiometric trigonal phase with a wide range of compositions. In this study, an almost stoichiometric phase, corresponding to the composition SnBi4Se7, has been identified and isolated. The structure has been determined by X-ray diffraction and the local environment of Sn atoms analyzed by Mossbauer spectroscopy. This phase has a rhombohedral structure, space group R (3) over bar m, with hexagonal lattice parameters a = 4.1602(5) Angstrom and c = 38.934(3) Angstrom. The unit cell contains three slabs, each one composed of seven atomic layers according to the sequence Se-Bi-Se-(Bi,Sn)-Se-Bi-Se. Bismuth atoms partially occupy two sites, 3a (0, 0, 0) and 6c (0, 0, z) with z = 0.4285, while tin atoms partially occupy only one site, 3a. Selenium atoms are placed in two different 6c sites, with z(1) = 0.1350 and z(2) = 0.7108.