Abstract Adding metals to carbon vapor leads to the formation of a rich variety of molecular structures. Examples include endohedral fullerenes, metallocarbohedrenes, metal–carbide nanocrystallites and perhaps most spectacularly, single-wall carbon nanotubes (SWNT). We discovered that pure carbon SWNT catalytically self-assemble when cobalt is co-vaporized with carbon in a fullerene generator. Third elements added to the metal and carbon can have important effects. Sulfur and bismuth can greatly increase the production efficiency of SWNT, leading to aggregation of SWNT into nanotube bundles or ropes, and can significantly extend the tube diameter distribution. Tubes larger than 5 nm were produced and S increased the mean tube diameter by ∼60%. A different type of ‘third atom’ effect was recently found at Virginia Tech: adding nitrogen to a scandium/carbon vapor leads to the formation of Sc [email protected] 80, a new type of endohedral fullerene. Erbium can substitute for any of the scandium atoms and we have measured the emission spectra and lifetimes of Er m Sc 3− m [email protected] 80 ( m=1–3). These were the first emission lifetime measurements reported for light coming from species trapped inside fullerenes.