Phase relations in the systems PbS-Sb 2 S 3 -Bi 2 S 3 and PbS-FeS-Sb 2 S 3 -Bi 2 S 3 were studied using evacuated capsule technique, X-ray powder diffraction, reflected light microscopy, and electron microprobe analysis. Seven new phases found in the systems are designated as phases C, K-K Fe , X, Y-Y Fe , Z, W, and R for convenience in their descriptions. Phases C, K-K Fe , and R have compositions and crystal structures comparable to cosalite, kobellite, and bismuthian robinsonite, respectively. Others have no distinct natural counterparts.The cosalite-like phase (C) when synthesized has a composition of 2PbS.(Bi (sub 0.80) Sb (sub 0.20) ) 2 S 3 and is stable to 715 degrees + or - 5 degrees C. No Sb-free cosalite was observed in the system. Kobellite, on the other hand, was synthesized from a series of compositions from 6PbS.FeS.Sb 2 S 3 .2Bi 2 S 3 (K Fe ) to 3PbS.Sb 2 S 3 .Bi 2 S 3 (K).Phase X, 9PbS.11 (Bi (sub 0.64) Sb (sub 0.36) ) 2 S 3 , is the only ternary phase synthesized in the PbS-poor half of the system PbS-Sb 2 S 3 -Bi 2 S 3 . Phase Y ranges in compositions from 2PbS.(Sb (sub 0.50) Bi (sub 0.50) ) 2 S 3 (Y 1 ) to 2PbS.(Sb (sub 0.76) Bi (sub 0.24) ) 2 S 3 (Y 2 ). The Sb-rich end members form equilibrium assemblages with sulfantimonides, whereas the Bi-rich end members form equilibrium assemblages with sulfbismuthinides. Members of intermediate compositions exist in equilibrium with galena. Phase Z, a low-temperature form of phase Y, is stable below 407 degrees + or - 8 degrees C.Phase W has a composition of 3PbS.(Sb (sub 0.80) Bi (sub 0.20) ) 2 S 3 and exists in a very limited association with other phases in the system PbS-Sb 2 S 3 -Bi 2 S 3 . Phase R has a composition of PbS.(Sb (sub 0.50) Bi (sub 0.50) ) 2 S 3 and a range of solid solution toward robinsonite.The system PbS-FeS-Sb 2 S 3 -Bi 2 S 3 is characterized by the formation of a single-phase plane defined by Y Fe , 6PbS.FeS.2Sb 2 S 3 .Bi 2 S 3 , Y 1 , 2PbS.(Sb (sub 0.50) Bi (sub 0.50) ) 2 S 3 , and Y 2 , 2PbS.(Sb (sub 0.76) Bi (sub 0.24) ) 2 S 3 . In the region between 66.66 and 60.00 mole percent PbS, stable binary assemblages consisting of (K-K Fe ) and (Y-Y Fe ) solid solutions are: K Fe -K-Y 1 , K Fe -K-Y Fe , K Fe -K-C, K Fe -K-lillianite solid solution, Y 1 -Y Fe -K, Y 1 -Y Fe -K Fe , Y 1 -Y 2 -K, Y 2 -Y Fe -K, Y Fe -Y 2 -boulangerite, and Y 2 -Y Fe -2PbS.Sb 2 S 3 .