In previous work, we reported the growth of well-structured single-walled carbon nanotubes (SWNT) by CO disproportionation on CO:MO/SiO2 catalysts. We have proposed that high SWNT selectivity can only be obtained when most of the Co in the sample is interacting with Mo and forming a cobalt-molybdate surface layer. In this contribution, we report further evidence to the hypothesis by an investigation of Na-doped CO:MO/SiO2 catalysts. The effect of the sodium addition has been investigated by Raman spectroscopy, ultraviolet-visible diffuse reflectance spectroscopy, H-2 temperature-programmed reduction, and X-ray photoelectron spectroscopy. The results are compared to those obtained in a previous study on undoped CoMO/SiO2 catalysts. Significant differences are observed between the undoped and Na-doped catalytic systems. It has been found that the presence of sodium favors the dispersion of molybdenum, which tends to form sodium molybdate species over the silica support. At the same time, the formation of these sodium molybdate structures hinders the interaction between cobalt and molybdenum, preventing the formation of the cobalt molybdate species. This disruption has been found to be detrimental to the SWNT selectivity of the catalyst, giving further evidence to the link between the presence of this surface species and the selective SWNT growth. (C) 2003 Elsevier Inc. All rights reserved.