The ability to integrate carbon nanotubes, especially single-walled carbon nanotubes, seamlessly onto silicon would expand their range of applications considerably. Though direct integration using chemical vapor deposition is the simplest method, the growth of single-walled carbon nanotubes on bare silicon and on ultrathin oxides is greatly inhibited due to the formation of a noncatalytic silicide. Using X-ray photoelectron spectroscopy, we show that silicide formation occurs on ultrathin oxides due to thermally activated metal diffusion through the oxide. Silicides affect the growth of single-walled nanotubes more than multi-walled nanotubes due to the increased kinetics at the higher single-walled nanotube growth temperature. We demonstrate that nickel and iron catalysts, when deposited on clean silicon or ultrathin silicon dioxide layers, begin to form silicides at relatively low temperatures, and that by 900 °C, all of the catalyst has been incorporated into the silicide, rendering it inactive for subsequent single-walled nanotube growth. We further show that a 4-nm silicon dioxide layer is the minimum diffusion barrier thickness that allows for efficient single-walled nanotube growth.