Compounds with high fluorinating capacity are obtained when rubidium and cesium chlorides are fluorinated in a stream of elemental fluorine. Best results are obtained when the, temperature is kept in the range of 140–170°C. These products were first assumed to be polyfluorides, MF3, of Rb and Cs1. It was later assumed by various researchers2that the products are really tetrafluorohalates of Rb and Cs, formulated as MC1F4. One of the authors was himself involved in the problem of trying to establish the structure of these products. It is, however, a very difficult problem since pure products giving constant analytical results were not obtained. During the fluorination process, which is an exothermic one, enough heat is evolved to melt the reaction medium and the temperature rises above that needed to decompose the products. It is therefore impossible to obtain reliable analytical data, the products being a mixture of regular fluorides, MF, and the active, fluorinating, ones. No final proof has therefore been found for the C1F−4structure.