A series of mixed fluorides with general composition Ba 1 - x Er x F 2 + x (0.00=<x=<1.00) were prepared by a vacuum heat treatment of the binary fluorides and analyzed by powder XRD, which revealed the phase equilibrium in BaF 2 -ErF 3 system. The nominal compositions with x=<0.25 compositions exist as cubic fluorite type solid solution. A rhombohedral ordering is observed in the vicinity of 42mol% of ErF 3 in BaF 2 . A monoclinic phase with the composition Er 2 BaF 8 dominates the ErF 3 -rich compositions. The compositions for x>0.67 exist as a mixture of Er 2 BaF 8 and leftover ErF 3 , whereas the compositions for x<0.67 exist as a mixture of Er 2 BaF 8 and rhombohedral fluorite derived phase. The evolution of the various phases will be discussed in reference to the powder XRD data.