This paper describes the development of a nonintrusive flow visualization method for the quantitative study of the dynamic velocity and void profiles across the pipe cross section in slug flow. The method utilizes novel digital image analysis and computer graphics techniques. Slug flow in a 75 mm diameter, 10 m long Plexiglass pipe is recorded on video using super-VHS cameras and an audio-visual mixer. Sixty images are obtained every second and these are digitized and analyzed on a SGI' graphics workstation. Detailed information regarding the local velocity and void distribution is obtained from the analysis and is used to generate velocity and void profiles across the pipe at different distances into the slug. Results show that gas is released into the slug in the form of pulses of bubbles resulting in the existence of large, frothy, highly aerated structures in the mixing zone. The hydrodynamic boundary layer is destroyed at the slug front but begins to redevelop in the mixing zone. It becomes fully developed at the end of the mixing zone and the one-seventh power law profile for velocity profile is applicable. At the end of the mixing zone, the gas moves towards the top of the pipe and the void fraction distribution tends to a steady profile. A process dynamic model is developed to predict the variation of the average liquid holdup within the slug. © 1997 Elsevier Science Ltd.