The disproportionation of CO over Co−Mo/SiO2 catalysts with low Co/Mo ratios results in a high selectivity to single-walled carbon nanotubes (SWNTs) when the reaction is conducted between 700 and 950 °C in a flow of pure CO at a total pressure between 1 and 5 atm. A detailed characterization involving Raman spectroscopy, temperature-programmed oxidation (TPO), and transmission electron microscopy (TEM) shows that the product of this process has a high concentration of SWNTs. Depending on the temperature of operation, the selectivity and nanotube diameter can be varied.