Abstract The composition dependence of the cubic → tetragonal phase transformation for solid solutions of UC 2 with LaC 2 and with CeC 2 is reported. The existence of cubic phases down to room temperature is shown for ternary carbides within the composition limits La 0·10U 0·90C 2La 0·85U 0·15C 2 and Ce 0·16U 0·84C 2Ce 0·81U 0·19C 2. A disordered domain model is described to account for the failure of a tetragonal phase to nucleate on cooling from the melt. The variations of lattice parameter with composition for the cubic solid solutions are given. The appearance of UC from the cubic solid solutions on annealing shows that UC precipitation occurs above the β → α transition. Ternary f.c.c. solid solutions containing C 2 units have been shown not to occur when the overall carbon/total metal ratio falls below 2:1. In such preparations the phases in the melt are UC and the lanthanide carbide expected from the melt stoichiometry. The limiting solubilities of Ce in UC and of U in Ce 2C 3 are 4·5% and 5·5 atom % total metal respectively.