Abstract: The fluoride halides of barium and strontium BaFCl, BaFBr, BaFI, SrFCl and SrFBr, as well as some binary systems of them BaFCl_1-xBr_x (Ⅰ), BaFCl_1-xI_x (Ⅱ), BaFBr_1-xI_x(Ⅲ), SrFCl_1-xBr_x (Ⅳ), and Sr_1-xBa_xFBr(Ⅴ) were synthesized by the method of solid state reaction at high temperatures. Among them BaFIwas prepared by the reaction of NH₄Iwith BaF₂ at 400℃ for the first time.Phase diagrames and the subsolidus phase relationships of these binary systems have been studied by thermal analysis and by the method of lattice parameter determinations respectively. Ⅰ, Ⅲ and Ⅳ are systems of completely miscible solid solutions with a minimum melting point. Ⅱ and Ⅴ are systems of binary eutectic. There exists a correlation between the type of phase diagrames of these binary systems and the relative difference of c/a of the constituents, of which the system consists.