In this study, a hierarchal web of carbon micro and nanofibers was used as a precursor for the synthesis of a carbon molecular sieve (CMS). CMSs were prepared by thermal treatment of carbon fibers using a microwave heating device. The heating power and treatment time were optimized for the maximum performance of the prepared CMS for the separation of CO2 at low concentrations from the gaseous mixture of CO2 and air under dynamic (flow) conditions. Based on the experimental data, microwave power input of 240W and treatment time of 15 min were found to be suitable for the maximum uptake of CO2 by CMS. Adsorption breakthrough curves were obtained at different gas flow rates and CO2 concentrations. CMSs prepared from the hierarchal web of carbon micro and nanofibers were found to be superior to those prepared from ACF. The CO2 uptake was determined to be approximately 0.88 mg/g and 10 mg/g at concentrations of 500 ppm and 5000 ppm, respectively, in air. (C) 2010 The Institution of Chemical Engineers. Published by Elsevier By. All rights reserved.