The decomposition of ferrocene and the nucleation of iron catalyst nanoparticles for carbon nanotube (CNT) synthesis in a continuous gas phase process are studied. The thermal decomposition of ferrocene and nucleation of iron catalyst nanoparticles take place in a horizontal tube furnace within a temperature range of 300–1300°C and in a hydrogen atmosphere at atmospheric pressure (compare Figure 1). The iron nanoparticles act as a catalyst to form CNTs from decomposed methane or other carbon sources. The resulting CNTs agglomerate to form an annulus concentric within the reactor, which propagates down the reactor. To date few studies have examined the phenomena associated with the ferrocene breakdown, catalyst nucleation and growth within the reactor. These phenomena are critical to the quality and throughput of the CNT production, as the CNT diameter and chirality are coupled to catalyst diameter.