This paper focuses on the reactive chemical vapour deposition of composite B4C-SiC coatings onto porous graphite substrates from a BCl3−SiCl4−H2vapour phase. The mixed layer seems to exhibit promising properties such as oxidation resistance. However, its preparation as a thin film from the vapour is very difficult owing to the complexity of the system. The experimental parameters such as temperature, number of moles H2, and the amounts of BCl3and SiCl4in the reactive vapour phase are systematically discussed. The results are compared with a thermodynamic approach. Also, this paper reports investigations on boron and boron silicide coatings. Scanning electron microscopy, X-ray photoelectron spectroscopy and electron microprobe analysis were used to characterize the various coatings. The compositions, microstructures and properties such as oxidation behaviour of the coatings were also studied. Optimal experimental parameters of the process were reached in order to achieve a dense, adherent and continuous mixed B4C-SiC layer on bulk graphite substrates with a view to coating further carbon fibres and protecting them from oxidation in air at temperatures less than 1000°C.