A comprehensive study of the BaF₂–Ba₃(BO₃)₂ phase diagram has revealed a significant difference between the two intermediate phases Ba₅(BO₃)₃F and Ba₇(BO₃)_4−y F_2+3y . The latter exhibited (BO₃)³⁻ ↔ 3F⁻ anionic substitution which, unusually, strongly influences the solidus temperature. A comparison of the Ba₅(BO₃)₃F and Ba₇(BO₃)_4−y F_2+3y crystal structures, along with consideration of other compounds demonstrating (BO₃)³⁻ ↔ 3F⁻ isomorphism, allows for the disclosure of the mechanism of (BO₃)³⁻ ↔ 3F⁻ heterovalent anionic substitution in fluoride borates via [(BO₃)F]⁴⁻ tetrahedral groups being replaced by four fluoride anions. No exception to this mechanism has been discovered among all known phases with (BO₃)³⁻ ↔ 3F⁻ substitution.