Multiwall carbon nanotubes (MWNTs) were formed in supercritical toluene using ferrocene to promote their growth. The crude reaction product is comprised of a mixture of MWNTs and carbon nanofilaments. The morphological difference in the products, i.e. MWNTs versus carbon nanofilaments, appears to originate from the dependence of the growth mechanism on the particle size, with seed particles larger than ∼30 nm in diameter producing filaments and smaller particles producing MWNTs.