The high‐temperature, uranium monocarbide‐dicarbide immiscibility region was determined by examining the microstructures of specimens with compositions from , which had been prepared by decomposing mixtures containing the sesquicarbide at 1820°–2130°C. The low carbon miscibility boundary, which occurs between and (5.2 and 5.7% carbon), is nearly composition invariant between 1820° and 2070°C. The high carbon miscibility boundary occurs at about (7.7% carbon) at 1820°C and the solubility of the monocarbide in the dicarbide phase increases linearly with increasing temperature. The peak of the immiscible region occurs at about (5.7% carbon) and a temperature above 2070°C.