The results of a thermodynamic and experimental study of the codeposition of three elements, silicon, boron, and carbon, by a classical chemical vapor deposition technique in a hot-wall reactor are compared. The initial gaseous mixture consisted of methyltrichlorosilane, boron trichloride, and hydrogen. By the thermodynamic approach, the nonstoichiometry of boron carbide was described from B4C to B10C, and a comparison can be made between accurate thermodynamic calculations and experimental compositions measured at a given position on the substrate. The thickness and composition uniformity was also studied as a function of the inlet gas composition. The codeposits were first obtained on graphite, but the final purpose was to extend the process to composite materials. When fixed conditions of temperature (T = 1400 K) and total pressure (P = 0.395 atm) were employed, the influence of the total gaseous flux was shown to be great, especially in a hot-wall reactor. The total flux can change the deposition results because of either the depletion phenomenon or some departure from equilibrium.