The pyrolysis of organometallic precursors followed by a catalyst-assisted transformation of carbon into graphite is one successful route toward the high-yield production of aligned carbon nanotubes (CNTs) with well-defined dimensions and structure. In this work we have studied the particular growth mechanisms of multiwalled CNTs synthesized from FePc and FeCp2 as precursors in the pyrolysis process. TEM investigations including in situ heating experiments enabled us to directly observe the surface segregation of graphite at the expense of the encapsulated catalyst material. The observations suggest that the structural growth process is determined by the dissolution of carbon in the quasi-liquid catalyst particle, its diffusion through the particle, and its segregation in the form of graphene layers in the case of supersaturation. These processes were proved to occur via the formation of an intermediate iron carbide phase. The results also demonstrated how the increasing expulsory forces may cause the gradual displacement of the fluid catalyst during nanotube growth. (C) 2003 Elsevier Inc. All rights reserved.