The gas holdups of an air-acetic acid-terephtalic acid system in a pressurized bubble column of 0.3 m diameter and 6.6 m height was studied. The overall holdups and the distribution of local holdups in the chum-turbulent flow regime with a gas velocity up to 0.54 m/s and a system pressure 1.0 MPa were obtained using gamma-ray densitometry. These results are in agreement with the results obtained in the system by differential pressure methods. The effects of superficial gas velocity, static liquid height, concentrations of acetic acid, solid and antifoam agents on the overall gas holdup were investigated. The local gas holdup increases with increases of the axial height and superficial gas velocity, and there is a section of accumulation of foam in the upper section of the column. The results also show that both the static liquid height and concentration of acetic acid have no significant effect on the overall holdup. The superficial gas velocity and concentration of solids and of antifoam agents have a great influence on the gas holdup. A correlation for the estimation of the overall gas holdup was obtained, which is significant for the design and scale-up of oxidation reactor of p-xylene. (c) 2005 Elsevier Ltd. All rights reserved.