Carbon nanotubes are considered to be a promising option for hydrogen storage. However, there is a wide variation in the hydrogen storage capacity of CNT's reported by different researchers. Hydrogen storage depends on many factors, the diameter being one of them. Also for repeatability and development of a viable storage system, control of diameter is necessary during the formation process. A technique involving digital image processing has been developed that can be used to determine the distribution of diameter of carbon nanotubes. The nature shows a distribution similar to a random distribution that may help in characterizing nanotube samples. The use of image processing and statistical analysis can help in developing better CNTs. (C) 2004 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.