Abstract The crystal structures of the compounds R 3CuSiS 7 (R = Ce, Pr, Nd, Sm, Tb, Dy and Er) and R 3CuSiSe 7 (R = La, Ce, Pr, Nd, Sm, Gd, Tb and Dy) (La 3CuSiS 7 structure type, space group P6 3, Pearson symbol hP24) were determined by means of X-ray single crystal and powder diffraction. The R atoms are surrounded by distorted trigonal prisms. A triangular surrounding exists for the Cu atoms. A tetrahedral surrounding exists for the Si atoms. The S(Se)1 and S(Se)3 atoms are surrounded by tetrahedra also. Four cations surround every S(Se)2 atom.