Using an original technique, new data for the interfacial reaction in B-Al composites have been obtained. Two stages of the boron fibre/aluminium interface reaction were revealed. In Stage 1, due to the aluminothermal reduction of the oxide film on the boron fibre, fine-crystalline rhombohedral boron, alumina and dispersed aluminium borides are formed. The latter are formed by the reaction with the fine-crystalline boron and do not affect the fibre strength but can contribute to a more intensive interaction. In Stage 2, borides are formed by direct reaction with the fibres, thus seriously reducing their strength.