The decomposition of CH4 in the presence of C2H4 or C2H2 using a nanosized iron or nickel based catalyst with volume ratios of C2H4 or C2H2 to CH4 of 1.75:1 to 0.55:1 was investigated. The presence of C2H4 or C2H2 increased the conversion of CH4 to 3 to 5 times that with no C2H4 or C2H2; that is, it significantly increased the production rate of carbon from 20−30 to 45–75 g/gcat/h at 723–873 K. The online detection of the time dependent composition of the product gas and experiments in which the feeding sequence of CH4 and C2H4/C2H2 was changed allowed us to propose a mechanism for the enhanced decomposition as follows. First, the facile exothermic adsorption and decomposition of C2H4 or C2H2 provides the driving force for the in situ endothermic decomposition of CH4. Subsequently, the thermodynamically favored intermediate products of aromatics or poly aromatics are formed by the insertion of CH4 into adsorbed C2H4 or C2H2 fragments. Finally, the intermediate products are rapidly dehydrogenated on the hi...