This study concerns a three-phase rectangular air-lift reactor. The first part deals with the reactor's gas-liquid two-phase flow hydrodynamics. Parameters, such as the gas hold-up, the liquid velocity, the mixing parameters, and the solid hold-up, were quantified in the presence of two different plastic solids-a regular shape solid called PI, and an heterogeneous shape solid, P4. No fundamental influence of the solids on the studied parameters was noticed, except for the gas hold-up. Indeed, a bubble coalescence phenomenon was highlighted for high solid concentrations (epsilon(s) > 16%). Moreover, a comparison of these two materials led to the conclusion that P4 exhibited better hydrodynamic performances. In a second set of experiments, the oxygen mass transfer was characterised in three-phase flow, with both kinds of plastic materials. The k(L)a coefficient was deduced from a mass balance on the gas phase. It was found to be weakly influenced by the presence, the nature or the quantity of solid. (C) 2004 Elsevier B.V. All rights reserved.