A series of computational calculations to understand the gas-phase reactions of xylene, a typical feedstock for carbon nanotubes, were conducted. A xylene reaction model was combined with a soot formation model, allowing us to calculate the detailed xylene pyrolysis reactions as well as the growth of polycyclic aromatic hydrocarbons (PAHs) and soot. The model was validated against soot formation experiments that were conducted by other researchers under the conditions similar to CVD nanotube synthesis; their experimental results confirmed the computed soot yield and soot surface area to be reasonable. Our calculations showed that xylene and toluene were the major gas-phase species at temperatures lower than 973 K, implying that nanotubes were formed through the interaction between catalyst and xylene and/or toluene at these temperatures. At higher temperatures, however, a considerable amount of acetylene was found which likely enhances the growth of PAHs and soot as evidenced at 1373 K. (c) 2006 Elsevier Ltd. All rights reserved.