We describe, for the first time, the sequential structural changes of bundles of double-walled carbon nanotubes (DWNTs) as a function of heat treatment temperature. DWNTs (inner diameters >0.9 nm) are structurally stable up to 2000 degreesC. Above 2100 degreesC, the outer walls of adjacent DWNTs start coalescing into large diameter tubes. At temperatures higher than 2100 degreesC, three different types of structures were observed: (a) large-diameter DWNTs; (b) multi-walled carbon nanotubes (MWNTs), and (c) flaky carbons. The results demonstrate that DWNTs are much more stable than SWNTs, and their stability is comparable to that of MWNTs. (C) 2004 Elsevier B.V. All rights reserved.