[en] The method of differential thermal analysis has been used to investigate solid-liquid equilibria in the systems ClF₅-BF₃ and BrF₅-BF₃ in the whole range of concent-- rations of components. No formation of chemical compounds is found. BrF₅-BF₃ system refers to the series of simple systems of the eutectic type and is characterized by eutectics depleted in bromine pentafluoride content (less than 5 mol. %) with the melting temperature close to melting temperature of pure boron trifluoride. In the ClF₅-BF₃ system components are mutually insoluble in both liquid and solid states