In this Letter, scanning tunneling microscopy (STM) was used to study the sidewall fluorination and the reverse process of defluorination of single wall carbon nanotubes (SWNTs) and double wall carbon nanotubes (DWNTs). The same single SWNT was imaged in situ before and after defluorination to show the diameter changes and tube cuttings, and these confirm previously reported statistical shortening of SWNTs. In comparison, the STM image of annealed fluorinated DWNT reveals the inner tube to remain intact while the outer tube is cut. Finally, the ex situ Raman spectroscopy was used to confirm the fluorination and defluorination processes. (C) 2012 Elsevier B.V. All rights reserved.