Airlift column bioreactors are gas-liquid contact devices characterized by a rising channel and a down flow channel due to gas hold up differences in these two channels. Local gas hold up distribution strongly affects the overall gas-liquid flow dynamics in airlift columns. In this work, local gas holdup distributions in a draft tube airlift column covering both bubbly flow and churn-turbulent flow regimes have been studied using computed tomography (CT) technique as well as conventional techniques. The radial and axial evolutions of the gas holdup distribution will be discussed, together with the effects of superficial gas velocity and geometry parameters. The obtained gas hold up results will also be used to verify various empirical and semi-empirical correlations in the literature. Moreover, th e obtained gas hold up information, combined with liquid flow dynamic information reported in Luo and Al-Dahhan (2008a, b), forms a benchmark database for the design and scale-up of airlift column bioreactors and for computational fluid dynamic (CFD) modeling validations. & 2010 Elsevier Ltd. All rights reserved.