This paper presents an ab initio study of the B/C/Cl/H gas phase mechanism, featuring 10 addition-elimination reactions involving BHiClj (i + j <= 3) species and a first description of the chemical interaction between the carbon-containing and boron-containing subsystems through the three reactions BCl3 + CH4 (sic) BCl2CH3 + HCl, BHCl2 + CH4 (sic) BCl2CH3 + H-2, and BCl2 + CH4 (sic) BHCl2 + CH3. A reaction mechanism is then proposed and used to perform some illustrative equilibrium and kinetic calculations in the context of chemical vapor deposition (CVD) of boron carbide. Our results show that the new addition elimination reaction paths play a crucial role by lowering considerably the activation barrier with respect to previous theoretical evaluations; they also confirm that BCl2CH3 is an important species in the mechanism.