We report detailed studies of the distribution of pentagons on carbon nanotube tips. For a conical tube end, five pentagons are clustered to form a sharp tip. By examining the process of initial oxidation, we can identify the most strained pentagons on the tip. The pentagon distribution, which determines the pattern of oxidation, can be deduced with reasonable confidence from high-resolution transmission electron microscopy and computer-image simulation. (C) 1999 Elsevier Science B.V. All rights reserved.