Single-walled carbon nanotubes of uniform diameter were grown in cobalt-substituted MCM-41 molecular sieves templated with C12 and C16 alkyl chains to result in pore diameters of 2.6 and 3.3 nm, respectively. The narrow diameter distribution of the tubes grown was probed by Raman, UV-visible, and NIR spectroscopy, as well as by high-resolution transmission electron microscopy. Tube diameters have been observed to vary with the size of the Co clusters formed during carbon deposition, as measured by extended X-ray absorption fine structure (EXAFS). It is proposed that the diameter of the carbon nanotubes grown in MCM-41 catalysts is controlled by the size of the metallic clusters formed in the template. Because MCM-41 catalysts of different pore diameter form Co clusters of different sizes, this mechanism can be exploited to grow carbon nanotubes of uniform, preselected diameters.