The phase and the structure of polycrystal material, which was prepared by calcining the co-precipitated fluoride powder of Sr and La with a ratio of 0.69: 0.31 mixed with SrS powder, were investigated by X-ray diffraction and electron probe scanning. In the materials, three phases were found, they areSr0.69La0.31F2.31 solid solution phase with a pure SrF2 cubic structure whose lattice constant is 5. 843 A, SrS single phase with a NaCl cubic structure, and an unknown phase that is produced by the interaction of SrS and co-precipitated powder. It was also found that SrS has no influence on the structure of Sr0.69La0.31F2.31 solid solution.