The relation between anthropogenic emissions of CO2 and its increased levels in the atmosphere with global warming and climate change has been well established and accepted. Major portion of carbon dioxide released to the atmosphere, originates from combustion of fossil fuels. Integrated gasification combined cycle (IGCC) offers a promising fossil fuel technology considered as a clean coal-based process for power generation particularly if accompanied by precombustion capture. The latter includes separation of carbon dioxide from a synthesis gas mixture containing 40 mol% CO2 and 60 mol% H-2. A novel approach for capturing CO2 from the above gas mixture is to use gas hydrate formation. This process is based on selective partition of CO2 between hydrate phase and gas phase and has already been studied with promising results. However high-pressure requirement for hydrate formation is a major problem. We have used semiclathrate formation from tetrabutylammonium bromide (TBAB) to experimentally investigate CO2 capture from a mixture containing 40.2 mol% of CO2 and 59.8 mol% of H-2. The results shows that in one stage of gas hydrate formation and dissociation, CO2 can be enriched from 40 mol% to 86 mol% while the concentration of CO2 in equilibrium gas phase is reduced to 18%. While separation efficiency of processes based on hydrates and semi-clathrates are comparable, the presence of TBAB improves the operating conditions significantly. Furthermore, CO2 concentration could be increased to 96 mol% by separating CO2 in two stages. (C) 2011 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.