The invention is directed to a process for producing carbon nanofibers and/or carbon nanotubes, which process comprises pyrolyzing a particulate cellulosic and/or carbohydrate substrate that has been impregnated with a compound of an element or elements, the metal or alloy, respectively, of which is capable of forming carbides, in a substantially oxygen free, volatile silicon compound containing atmosphere, optionally in the presence of a carbon compound.