The phase diagram for the binary system has been obtained by correlating information from differential thermal analysis measurements, microscopic studies, and x‐ray powder patterns. In establishing the terminal points for this diagram, the melting point for was found to be 667° ±1°C. For , a melting point of 1092° ± 2°C was indicated. In going across the diagram, four peritectic transformations are apparent. The first is at 785°C and 50% in ; the second is at about 702°C and 80% in ; the third is at 695°C and about 94% in ; and the fourth occurs at 678°C and about 98% in . There is a large retrograde solubility of in . The ε‐phase transforms to an ε′‐phase at 550 °C in an apparent order‐disorder transition. Electrical measurements on some peritectic compositions from this diagram are reported elsewhere.