Alumina deposition from AlCl3-H2-CO2 mixtures at about 1300 K and 0.05 atm is investigated as a function of initial gas composition, from a thermodynamic point of view for two different substrates: a chemically inert substrate and a TiC substrate. Alumina is deposited with a 100% thermodynamic yield when water is formed in excess at equilibrium. Carbonalumina codeposits are obtained for hydrogen-rich initial compositions. Alumina is still deposited alone, but with low yields, when CO2-rich initial compositions are used with inert substrates. In contrast, under the same initial compositions, alumina is deposited by an associated reaction involving the substrate in the case of TiC. Experimental data that could support the occurrence of alumina-carbon codeposits and oxidation of TiC substrates are discussed.