The influence of the CO pressure and of the duration of catalyst exposure to CO was systematically investigated with respect to the carbon yield, selectivity, and diameter uniformity of single wall carbon nanotubes (SWNT) produced by CO disproportionation over Co-MCM-41 catalysts. The SWNT were characterized by Raman and near infrared (NIR) spectroscopy, while the state of the catalyst and the size of the metallic cobalt clusters were investigated by X-ray absorption spectroscopy (XAS). The experimental results suggest that both the selectivity to SWNT and the uniformity of their diameters are controlled by the relative rates of the following competing processes: cobalt reduction, nucleation of cobalt into clusters, and SWNT growth. It was found that reaction at low CO pressures leads to a wider distribution of diameters and lower SWNT selectivity. The duration of catalyst exposure to CO was found to strongly affect the SWNT selectivity at the early stages of SWNT growth, while it has a minor effect on the SWNT diameter uniformity. (C) 2004 Elsevier Inc. All rights reserved.