The existence of a critical temperature T-c in the chemical vapor deposition growth of carbon nanotubes, including single-walled nanotubes, is shown by in situ optical imaging of growing nanotube forests. At T-c, the temperature dependence of the chemical processes and characteristics of the grown nanotubes change. The lifetime and initial growth rate show this critical temperature and are linked by a characteristic energy through the initial growth rate decay. Ex situ Raman spectroscopy shows that the number of radial breathing modes, G band, and D band show the same characteristic energies and T-c. Crown Copyright (C) 2009 Published by Elsevier B. V. All rights reserved.