Catalytic chemical vapor deposition (CVD) is a popular method to synthesize carbon nanotubes (CNTs). At the presence of catalysts (usually trasition metals), the hydrocarbon feedstock decomposes controllably at elevated temperatures and can form tubular structures. It has been suggested that trace amounts of weak gas-phase oxidants, such as CO2, can enhance the CNT synthesis by extending the catatlyst life. It is not clear, however, how such additives affect the CVD reaction environment. In this study, ethylene gas was introduced to a preheated furnace/CVD reactor where meshes of stainless steel were placed. Therein ethylene was thermally decomposed in nitrogen mixed with different amounts of carbon dioxide. The meshes served as catalytic substrates for the CNT growth. The compositions of the ethylene pyrolyzates were analysed both with and without the presence of catalysts, to explore the possible contributions of CO2 addition to the CNT formation. The latter compositions were compared with kinetic model predictions of the thermal decomposition of ethylene. Both experimental and simulation results indicated that 1,3-butadiene (C4H6) was the most abundant hydrocarbon species of ethylene decomposition (at 800 °C) and that decomposition was inhibitted at the presence of CO2. A commesurate effect on CNT formation was observed experimentally, whereas the quality of CNTs got improved.