Abstract. A portion of the liquid-solid phase equilibrium region of the system U-UBr3 has been investigated by the method of thermal analysis. Uranium tribromide was prepared by introducing bromine vapor into a bulb containing finely divided uranium formed from the hydride. Cooling curves were obtained with tribromide and then with various compositions of tribromide and uranium. the information obtained showed that no solid sub halide exists but that the metal was soluble in the halide.