The crystal structures of the 4:3 stoichiometry compositions in the rare earth-antimony and bismuth systems have been studied. The Gd4Bi3 (anti-Th3P4) structure was formed only by the lighter elements, La to Dy. This structure has only been observed for compounds with radius ratios, based on ionic radii, between 0.417 and 0.520. The lattice constants of most of these compounds are linearly dependent on the trivalent ionic radii of the rare earths. Deviations from this linear dependence were observed for compounds containing Eu, Sm and Yb, which suggest that these elements are, at least partly, in the divalent state. © 1967.