A gamma ray tomography system was designed, developed and employed for the measurement of radial hold-up profiles in bubble columns. Measurements were of gamma ray attenuation. For the given source strength the collimator designs for both source and detectors were optimized to Set the maximum number of counts with the least scattering effects. For the given dwell and scanning times, excellent reproducibility was obtained. Both fan beam and pencil beam scanning were employed to minimize the error due to scattering of the photons near to the wall regions. Experiments were performed in a 0.385 m internal diameter bubble column. Chordal hold-up profiles were used to find the radial hold-up profile using Abel's inversion method. (C) 2001 Elsevier Science B.V. All rights reserved.