During the preheating treatment preceding the processing of C/Al composites, a decrease in the mechanical properties of the carbon fibres is generally observed, owing to the oxidation of the fibres. Thus, it implies the use of a surface coating likely to resist the oxidation of the fibres. In this work, we chose to protect the carbon fibres by a B4C coating, using a RCVD process. This coating leads to an effective protection against oxidation, as was shown by the mechanical properties obtained by single fibre tests on the coated fibres before and after preheating. However, although this coating also protects the fibres against the reaction with liquid aluminium during processing, the tensile properties of the composite were very weak. An analysis of these results based both on the use of loose bundle tests and on a microstructural characterization of : the reinforcement and of the composite by transmission electron microscopy (TEM) observations and by electron spectroscopy for chemical analysis (ESCA) led us to the conclusion that the local overthicknesses of the B4C coating have an embrittling effect, leading to a premature failure of the composite.