Two interesting carbon nanostructures, the double helices and the nanobraids, have been synthesized by pyrolysis of acetone at 715 degreesC, using iron nanoparticles as catalysts. The double helix was formed by two nanofibers, strictly coiled together with a strikingly constant pitch. Such a rope-like structure usually extended hundreds of microns in length, and provided excellent strength, stability and flexibility. The nanobraids appeared to be partially rolled up from carbon layers. Electron microscopy was applied to characterize these carbon forms. (C) 2002 Elsevier Science Ltd. All rights reserved.