Abstract The lanthanoid mixed halide systems, MCl 3 MBr 3, for M = Gd, Tb, and Yb, have been prepared by mixing and fusing the pure reactants and have been examined by X-ray powder diffraction procedures. For M = Gd, UCl 3, ( P6 3 m )-, PuBr 3 ( Cmcm)- and AlCl 3 ( C2 m )- type solid solution regions were found. For M = Tb, PuBr 3- and AlCl 3-type solid solution regions were found. For M = Yb, AlCl 3 − and FeCl 3-(R 3)- type solid solution areas were observed. Pure TbBr 3 appears to be isostructural with GdBr 3 rather than with DyBr 3. These systems are discussed and compared to related systems.