To elucidate the growth mechanisms of carbon nanotubes by chemical vapor deposition, we performed in situ mass spectroscopy for methane CVD. Methane is considered stable at temperature below 1000 degrees C; however, using appropriate catalysts, it can be decomposed. In previous studies, acetylene and ethane were thought to be key intermediates in the synthesis of carbon nanotubes, but in this study, ethylene was found to be the key building block. The yield of ethylene was low but this is consistent with the fact that the production rate of carbon nanotubes by methane CVD was lower than that by ethylene CVD synthesis. (C) 2012 Elsevier B. V. All rights reserved.