Ternary phases in the system Cu-As-S are luzonite, enargite, tennantite, lautite (CuAsS), sinnerite (Cu 6 As 4 S 9 ) and a new compound A (Cu 24 As 12 S 31 ). The only compound to have an extensive composition field is tennantite, for which the generalized formula Cu (sub 12+x) As (sub 4+y) S 13 is proposed, where 0 < or =x< or =1.72 and 0 < or =y< or =0.08. Down to 300 degrees C the lowest temperature studied, the temperature sensitive composition field for tennantite does not reach the generally accepted composition Cu 12 As 4 S 13 . It is highly probable that this composition is reached at lower temperatures.Phase relations are dominated by the join Cu 2 S-As, which remains stable from the liquidus down to at least 300 degrees C and precludes the stable coexistence of the copper arsenides with ternary compounds, and by the expansion of a liquid field from the As-S sideline into the ternary. Interactions between the solids, and the liquidus relations occasioned by the expanding liquid field, lead to a series of reaction points of interest in the studies of ore deposits.