The carbon nanotubes (CNTs) formation by catalytic chemical vapor deposition is initiated by precursor decomposition to form a multitude of reactive species. During large-scale CNT self-assembly, these species vary with residence time leading to a non-uniform CNT growth. This Letter studies the self-assembly, and the reaction pathways leading to CNT formation. A tubular plug flow reactor, where CNT deposition occurred at various residence times was used to study the process, and kinetic simulation was used to predict the reaction pathways. There was excellent agreement between experimental and the modeling results, providing an insight into the mechanism of CNT growth. (c) 2006 Elsevier B.V. All rights reserved.