A portion of the ternary system EuEu 2O 3EuF 3 has been investigated by equilibrating EuOEuF 3 and EuOEuF 2 mixtures at 1500°C and examining the products with a polarizing microscope and by X-ray diffraction. The composition at the liquid-solid boundary was not measured directly, but has been estimated. A phase diagram is presented for that part of the system studied. No ternary compounds are formed with europium in an oxidation state lower than +3 in samples cooled from 1500°C. Several compositions of the Eu 2O 3Euf 3 and Sm 2O 3SmF 3 binary systems were examined using differential thermal analysis and X-ray powder diffraction. In these systems, at least one intermediate phase (and probably two or more) in addition to LnOF is stable. These additional phases all lie on the fluoride side of LnOF. At temperatures above 600°C, solid solutions extend from approximately LnO 0.8F 1.4 to LnO 0.6F 1.8.