Phase relations have been determined for the system Cu-Sb-As-S between 350 degrees C and 500 degrees C. A new compound, Cu 4 (As,Sb) 2 S 5 , the only quaternary phase synthesized in this study, has not yet been reported as a mineral. It is not stable in either the Cu-Sb-S or Cu-As-S ternary system yet is ideally a ternary compound because Sb and As replace each other freely by solid solution. The compound ranges in composition from Cu 4 (Sb (sub 0.26) As (sub 0.74) ) 2 S 5 to Cu 4 (Sb (sub 0.55) As (sub 0.45) ) 2 S 5 . The d values (A) and intensities of the four strongest X-ray diffraction lines are 2.99(10), 2.59(4), 1.83(8), and 1.56(7) for the composition Cu 4 (Sb (sub 0.391) As (sub 0.609) ) 2 S 5 . The X-ray pattern resembles that of tennantite in its strongest lines. Because the phase is demonstrably stable down to 350 degrees C, and is apparently stable to even lower temperatures, it is probable that it will be discovered as a mineral.Phase relations are controlled by extensive substitutions of As for Sb in phases that appear on the ternary faces Cu-Sb-S and Cu-As-S. The important compounds are enargite, Cu 3 (As,Sb)S 4 ; famatinite, Cu 3 (Sb,As)S 4 ; luzonite Cu 3 (As,Sb)S 4 ; chalcostibite, Cu(Sb,As)S 2 ; high-skinnerite, Cu 3 (Sb,As)S 3 ; compound A, Cu 24 (As,Sb) 12 S 31 ; and the solid solution series tennantite-tetrahedrite, Cu (sub 12+X) (As,Sb) (sub 4+Y) S 13 . The composition volume for tennantite-tetrahedrite is complex and continuous from the pure As to the pure Sb end-member compositions. Compositions in the quaternary are always less sulfur rich than the ideal formula Cu 12 (As,Sb) 4 S 13 .Besides extensive substitution of As for Sb, the dominant controls on phase relations are (1) a set of tie lines between Cu 2 S and the continuous As-Sb binary solid solution and (2) a large liquid field centered on the ternary face Sb-As-S that extends far out into the quaternary.