The physisorption and chemisorption of O-2 on the inner and outer walls of a (9,0) carbon nanotube have been studied using the ONION! method and the MP2 correlation treatment which includes long-range dispersion interactions. We have compared our results with previous theoretical work using density functional theory and with experiment. Physisorption occurs both inside and outside the tube but the binding is weak and very little charge transfer occurs. Chemisorption on the outer wall is energetically favorable but has a very high barrier and is not likely to occur under normal conditions. Dangling bonds must be present at the surface for O-2 to react. (C) 2002 Elsevier Science B.V. All rights reserved.