The phase diagram gadolinium fluoride - lutetium fluoride was determined by differential scanning calorimetry (DSC) and X-ray powder diffraction analysis. Both pure components undergo a reversible first order transformation to a high temperature phase. The mutual solubility of both components is unlimited in the orthorhombic room temperature phase. The maximum solubility of Lu in the high temperature phase of GdF3 (tysonite type) is about 20% and the maximum solubility of Gd in LuF3 ($��$-YF3 type) is about 40%. Intermediate compositions of the low temperature phase decompose upon heating in a peritectoid reaction to a mixture of both high temperature phases.

6 pages, 4 figs., accepted for J. Solid State Chem