Temperature and CH4/H-2 ratio of gas-flow rates are the two factors that strongly affect the qualities of vertically aligned single-walled carbon nanotubes (SWCNTs) in gravity-assisted chemical vapor deposition (CVD). The qualities of SWCNTs and other carbon products grown by gravity-assisted CVD were characterized by scanning electron microscopy and Raman spectroscopy. At temperatures between 850 and 900 degrees C, SWCNTs of very good quality stand alone on the substrate. At other temperatures, nanofibers or irregular islands of carbon are present on the substrate. The CH4/H-2 ratio influences the quality of SWCNTs more abruptly than temperature. At low ratio, no carbon nanotube is formed. The window of CH4/H-2 ratio for the growth of vertically aligned SWCNTs ranges from 160:80 to 160:40. At a ratio higher than 160:40, multiwalled CNTs replace SWCNTs and become the dominant product. (C) 2007 The Electrochemical Society.