Multiwall carbon nanotubes (MWNTs) were synthesized in supercritical toluene at 600degreesC and similar to12.4 MPa using ferrocene, Fe, or FePt nanocrystals as growth catalysts. Toluene serves as both the carbon source for nanotube formation and the solvent. In contrast to vapor-phase synthetic routes, the supercritical solvent provides high precursor concentration and a homogeneous reaction environment with dispersed growth catalyst particles. Both carbon filaments and MWNTs are produced by this approach, and a growth mechanism is proposed to explain the factors that determine the nanotube versus filament morphology. The plasmon energies of the pi and pi + sigma valence electrons were measured using electron energy-loss spectroscopy (EELS) of individual carbon fibers and MWNTs as a characterization tool to complement the imaging data obtained from electron microscopy.