The ternary system U-C-0 was investigated and a diagram is given expressing the phase relations in specimens which were reacted a t 1800'C in vacuum or in inert atmospheres and furnace cooled. A U(C,O) single-phase area exists in this diagram. T h e limit of substitutional solid solution of oxygen atoms for carbon atoms in UC is 12.5 at.%, which means that only 25% of the carbon atoms in UC can be replaced by oxygen atoms. The oxygen-saturated U C phase, U(CO.~,OO.Z,), has a lattice parameter of 4.953 f 0.001 A. The carbon monoxide partial pressures in equilibrium with this phase were determined for temperatures between 1705' and 1855'C.