The plunging liquid jet bubble column is an effective device for gas-liquid contacting. Small bubbles are formed in a high-shear region surrounding the plunging jet, leading to high interfacial area per unit volume of gas. At the same time, the counter-current flow downstream of the jet leads to regions of high gas holdup, producing high interfacial area per unit volume of reactor. This paper presents a study of the void fraction in the pipe-flow zone of the downcomer in a plunging jet reactor. It was found that the Ergun equation, using the standard constants derived from data for pressure drop in packed beds of solids, successfully predicted the gas void fraction for both bubbly and churn-turbulent flow conditions provided the increase in bubble size, with increasing gas input, was taken into consideration. Drift-flux analysis was also applied to the pipe-flow zone, and highlighted the transition from bubbly to churn-turbulent flow and a maximum gas void fraction operating condition of approximately 0.55. From the analysis the distribution coeffient for the downflowing system was found to be in the range 0.99-1.04, which was consistent with the measured radial void fraction profile and developed pipe flow for the two-phase mixture. (C) 2003 Society of Chemical Industry.