Direct growth of carbon nanotubes (CNTs) on silicon is of great importance for their potential exploitation in the semiconductor industry. In this article we investigate the chemical vapor deposition growth of CNTs on Si substrates from ethylene precursor using an iron catalyst. We observe that CNTs are produced only at temperatures between 830 and 980 °C, and within this narrow temperature window CNT yield initially increases with temperature to reach a maximum around 900 °C and then declines. While the requirement of a minimum temperature to initiate CNT growth can be understood by considering the minimum energy necessary to activate the catalyst particles, characterization of the as-grown CNTs by atomic force microscopy and x-ray photoelectron spectroscopy reveals that a loss of catalyst is responsible for the observed decline in CNT yield above 900 °C. However, unlike some previous reports suggesting surface silicide formation as the mechanism for catalyst loss, we find that either subsurface diffusion or evaporation is the mechanism for the loss of catalyst material in the current study.