Hydrodynamics and gas-liquid mass transfer in a rectangular airlift loop reactor with an internal baffle has been studied in this work. Measurements were carried out for two- and three-phase systems using air-water and air-water-polystyrene particles. The effects of the superficial gas velocity and solid loading up to 15% (v/v) were examined and correlations describing the hydrodynamics and gas-liquid mass transfer behaviours were proposed. The experimental results of mass transfer were compared with those of the literature. It was found in all cases that the gas hold-up in the riser and downcomer and volumetric mass transfer coefficient increased with increasing the superficial gas velocity, while the liquid velocity in the downcomer increased with U-gr until 0.033 m s(-1) and then decreased to reach a plateau value for Ugr > 0.04 m s(-1). It was also found that the liquid velocity and the volumetric mass transfer coefficient decreased only with the initial addition of solid for U-gr higher than 0.02 m s(-1) and that the gas hold-up in the downcomer was significantly influenced by the presence of solid for a concentration above 10% (v/v), while the gas hold-up was almost not affected in the riser. The aeration efficiency was also calculated and was found to be in the range 2.54-4.41 kg O-2/kW h. (c) 2005 Published by Elsevier B.V.