Gamma-densitometry tomography (GDT) experiments have been performed to measure gas holdup spatial variations in two bubble columns : a 0.19 m inside diameter Lucite column and a 0.48 m inside diameter stainless steel vessel. Two-phase air/water flows were examined. Horizontal scans at one vertical position in each column were made for several air flow rates. An axisymmetric tomographic reconstruction algorithm based on the Abel transform has been used to calculate the time-averaged gas holdup radial variation. Integration of these profiles over the column cross section has yielded area-averaged holdup results, which have been compared with volume-averaged gas holdups determined from differential pressure measurements and from the rise in the air/water interface during gas flow. The results agree reasonably well.