The axial distribution of gas holdups is measured using a gamma-ray densitometry in the pressured bubble column of 0.3 m diameter and 6.6 m height. The principle of gamma-ray measurement and data processing is discussed. The axial and average holdups in the two-phase system are obtained in the chum-turbulent flow regime with a gas velocity up to 0.40 m s(-1) and a system pressure up to 1.0 MPa, which are in agreement with results obtained by a conventional method (differential pressure measurement along the column height). The effects of superficial gas velocity, liquid surface tension, liquid viscosity, and system pressure on the axial gas holdup are investigated in this study. The axial gas holdups decrease with the increase of liquid viscosity and liquid surface tension, and increase with the increase of pressure and superficial gas velocity. Furthermore, it is demonstrated that gamma-ray attenuation method can be used to quantify the gas distribution and fingerprint the characteristics of the flow within such a reactor. The methodology proposed here could be also used as a tool to quantify and optimize the performance of other types of complex reaction systems. (c) 2005 Elsevier B.V. All rights reserved.