A kinetic study of the synthesis of carbon nanotubes (CNTs) by catalytic decomposition of acetylene over an iron-supported catalyst was carried out using a thermobalance. The CNTs were mainly characterized by transmission electron microscopy (TEM and HRTEM). It was found that there is a high influence of the reaction conditions (feed mixture, reaction temperature, etc.) on the final reaction yield and on the structural and morphological characteristics of the carbon products obtained. A complete discussion of the results by comparison of the carbon deposition curves with the TEM images is given. Finally, a kinetic model for explaining the growing mechanism of carbon nanotubes is developed and the displayed conclusions are clearly shown. (C) 2004 Elsevier Inc. All rights reserved.