Catalyst pretreatment and CO disproportionation reaction conditions were observed to strongly affect the diameter uniformity of single-wall carbon nanotubes (SWNT) grown on Co-MCM-41 catalysts. The prereduction and CO disproportionation reaction temperatures were varied systematically while the carbon loading and the SWNT diameter uniformity were monitored by TGA, Raman spectroscopy, and TEM. The state of the catalyst during prereduction and the size of the cobalt clusters formed during the SWNT growth process were monitored by in situ XANES during the prereduction of the Co-MCM-41, and ex situ EXAFS of catalyst samples was performed after carbon deposition. These experiments allow development of correlations between the SWNT quality and the state of the catalyst. Control of the cobalt cluster size in the Co-MCM-41 catalyst is critical to the SWNT diameter control. The size of the cobalt cluster changes with both the prereduction and the SWNT synthesis temperatures. SWNT with a very narrow diameter distribution can be grown in Co-MCM-41 by controlling both the prereduction and the reaction temperatures. (C) 2004 Elsevier Inc. All rights reserved.