The lattice parameters and atomic positions of the atoms, in particular the M-atoms, for arc-melted Sm 2 Fe 1 7 - y M y compounds with M=Ga, Al, Si and y=0, 1, 2 were determined using X-ray diffraction methods and subsequent Rietveld refinement. An increasing unit cell volume was found for M=Ga and Al, and a decreasing cell volume for substituting Fe by Si. Ga occupies the atomic position 18h in the rhombohedral R-3m Zn 2 Th 1 7 -type structure in all cases, whereas Al and Si occupy the 18h position up to a concentration of 1 atom per formula unit and for a larger concentration, additionally, the 18f position. Despite of the different behaviour of the lattice constants, the nearest neighbour distances were found to behave similarly for all substituting elements in Sm 2 Fe 1 7 - y M y because of different shifts of the atomic sites.