This paper reports preliminary results of a study on the hydrodynamics of a two-dimensional slurry bubble column. Experiments have been carried out with air/paraffin oil slurries with solids concentrations of 0, 28.3 and 38.6 vol% of porous silica particles (mean diameter of 38 mu m). Bubble sizes, bubble coalescence and bubble break-up rates were determined by video image analysis. Increasing slurry concentration increases the size and size distribution of the "large" bubbles, defined here as having diameters larger than 10 mm. Increasing slurry concentration reduces the total gas holdup to a significant extent; this reduction is to be largely attributed to the destruction of the "small" bubble population, which have bubble diameters smaller than 10 mm. Video imaging experiments lead to new insights into the mass transfer mechanisms from "large" bubbles. These "large" bubbles are continually coalescing and breaking up. The coalescence and breakup rates were determined by a frame-by-frame analysis of the video recordings and found to be at least 4 s(-1). A population model for mass transfer has been set up and used to establish that frequent bubble-bubble interactions could lead to an order of magnitude increase in the mass transfer rates for the large bubble class.