In order to clarify the origin of the side chain substituents on the benzene ring of aromatic of jet fuel, H2 was replaced by D2 and the origin of the substituents was traced using the isotope D atom. When the mixture of CO2/H2/toluene was used as the reactant, the mass spectra of toluene, ethyltoluene and (iso)propyltoluene were consistent with the standard spectra (Fig. ). When H2 is replaced by D2, the mass-to-charge ratio (m/z) due to toluene moves from 91 to 96 (Fig. ), indicating that H on benzene ring instead of methyl group could be readily substituted by D. The m/z attributed to the m-xylene ionic fragment and the 3-ethyltoluene molecular ionic fragment originating from ethyltoluene also moved from 105 and 120 to 111 and 129 (Fig. ), respectively, indicating that the H on the benzene ring and the H on the ethyl side chain of ethyltoluene were readily substituted by D, but the H on the benzene ring methyl group remained difficult to substitute by D. The other two main products, propyltoluene and isopropyltoluene, exhibited similar phenomena (Fig. and 5d), which well proved that the methyl group of toluene was not broken, then the ethyl and (iso)propyl side chains in both ethyltoluene and (iso)propyltoluene were produced from CO2 and H2.