A computed tomographic scanner was developed for imaging gas-holdup distributions in two-phase flow systems such as bubble columns and fluidized beds. The scanner has been wed to study, the effects of various operating parameters (such as column diameter, superficial gas velocity, and distributor type) on the gas holdup and its distribution in an air-water bubble column. The experimental investigation shows that the column dimensions have no significant effect on the vein fraction when the column diameter is greater than 0.15 m. Differences in the holdup distribution due to the kind of distributor used are significant only at low gas flow rates. Surface tension of the liquid has a profound influence on the gas holdup and its distribution.