This paper studies the effect of chromium addition to the Co-MCM-41 catalyst in the synthesis of single wall carbon nanotubes (SWNT). The molecular ratios between the two metals have been varied in the catalyst template and their effect on the synthesized SWNT distribution. has been studied. By adding Cr to the Co-MCM-41 monometallic catalyst, the diameter distribution of the synthesized SWNT has shifted towards smaller diameter tubes. This shift was correlated with the development of a bimetallic oxide in the MCM-41 framework during catalyst synthesis. By use of fluorescence spectroscopy, the relative abundance of certain tube chiralities has been seen to increase in the bimetallic samples compared to the monometallic ones as for example the (6,5) nanotube. X-ray absorption analysis of the catalyst before, during and after the SWNT synthesis, suggested that the use of a less reducible oxide (chromium oxide) to anchor clusters of a nanotube growth catalyst (cobalt clusters) is an important general tool for engineering the resultant nanotube properties. The addition of chromium has been seen to affect both the reducibility of the cobalt ions and the size of the resultant particles during the SWNT synthesis process. (C) 2009 Elsevier B.V. All rights reserved.