A comparative study was carried out of the process of plasma chemical deposition of boron carbide from hydrogen plasma containing the mixtures of BF3 + CH4 and BCl3 + CH4 sustained by RF arc (13.56 MHz) discharge. It was shown that in the case of synthesis of B4C from a mixture of BF3 + CH4, carbon and complex coordination compound [X3B]H--(+) (R3B center dot FH) are formed as the by-products of condensed products. In the case of synthesis of B4C from the BCl3 + CH4 mixture, the only condensed product is carbon. Mechanisms for the formation of boron carbide on the surface of heated electrodes are proposed. The main feature of these mechanisms is the preliminary deposition of a graphite layer from CH4 and then the precipitation of boron with the participation of the radicals BF2, BF and BCl. B4C samples were obtained and the impurity composition, morphology and structure of bulk boron carbide samples obtained using both of its halides were studied. It was found that in both cases a carbon phase is present in boron carbide samples. The main impurities entering the B4C, in the case of using a mixture of BF3 + CH4, is silicon, and in the case of a mixture of BCl3 + CH4, is tungsten.