Growth enhancers, most notably water, have been reported to increase catalyst activity, despite constituting 0.02% of the growth ambient, resulting in dramatic increases in vertically aligned carbon nanotube (CNT) forests. We demonstrate a surprising second function of the growth enhancer, i.e., its ability to control the structure of the CNTs exemplified by highly selective double-walled carbon nanotube synthesis using oxygen-containing aromatics. This approach provides an easy and convenient method to grow various kinds of nanotubes from the same catalyst and same growth conditions.