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Direct and non-reductive conversion of CO2and ethanol to form diethyl carbonate (DEC) is limited by its severe equilibrium and typically necessitates a dehydrant to shift the equilibrium toward the product side. We previously found the combination of two additives, 2,2-diethoxypropane (DEP) and H-FAU, to function as an effective dehydrating system and accelerate the DEC synthesis, although their actual function has remained unclear. In this study, the roles of DEP and H-FAU and the whole catalytic mechanism were investigated by a detailed kinetic study and diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy. Our results have suggested that an ethyl carbonate species is formed on the CeO2surface from CO2and either ethanol or DEP. Meanwhile, DEP is not involved in the rate-determining step and instead readily adsorbed and activated on the H-FAU surface. An ethanol molecule behaves as another ethoxy source in the DEC synthesis. Altogether, the ethyl carbonate adspecies on CeO2, the activated DEP species on H-FAU, and the ethanol molecule have been suggested to participate in the rate-determining step of the DEC synthesis.