The colloidal properties of catalysts present in three phase slurry reactors may affect the overall behaviour of such a reactor. In part, this may arise from the adhesion of such particles to gas bubbles, which may lead to enhancement of the G-L mass transfer [Catal. Today 48 (1999) 131]. Classical film theory based on the mass transfer models are capable of quantitatively predicting such mass transfer enhancement, but only on the basis of a priori knowledge of the degree of bubble coverage by particles. Presently, various methods are reviewed that allow the accurate experimental determination of the coverage of bubbles by particles, under both stagnant and non-stagnant conditions. The relevance of these phenomena to reactive systems will be demonstrated on the basis of mass transfer data for the hydrogenation of methyl acrylate in a slurry reactor.