A transient kinetic study of the reactions of C2H4 and C2H6 with lattice oxygen, O2- (in the absence of gaseous oxygen), and "adsorbed" oxygen (in the presence of gaseous oxygen) species over undoped and Li+-doped TiO2 OCM catalysts was performed. The results obtained indicate that lattice oxygen is more reactive towards C2H6 than C2H4 to form mainly CO2, whereas "adsorbed " oxygen exhibits higher reactivity towards C2H4. Incorporation of Li+ cations into the lattice of TiO2 affects significantly the reactivity of lattice oxygen towards C2H6 and that of "adsorbed" oxygen species towards both hydrocarbons. The interaction of C2H6 and C2H4 with the lattice oxygen of the catalysts results in the accumulation of carbonaceous species of different amounts and compositions under the same experimental conditions. Oxidation of these carbonaceous species to CO2 follows the same mechanism with at least two steps controlling the overall process. The activation energy of these steps is larger in the case of carbonaceous species derived from C2H4 than in the case of those derived from C2H6.