The effect of partitioning plates in the riser of external-loop airlift reactors (ELAs) was studied to increase the gas holdup by the restraint of bubble coalescence and the control of the liquid circulation velocity, and to improve the mass transfer in highly viscous liquids. The liquid circulation velocity, the gas holdup in the riser and the volumetric mass transfer coefficient were measured by changing the following parameters, that is, the number of stages, opening ratio of partitioning plates, diameter of a perforated hole and physical properties of liquids. The following conclusions were obtained; the liquid circulation velocity in the riser decreases with the increased number of stages. The gas holdup in the riser is affected by the liquid circulation velocity and bubble coalescence caused by partitioning plates. The gas holdup in the riser and the volumetric mass transfer coefficient increase with the increased number of partitioning plates. In highly viscous pseudoplastic solutions, the mass transfer rate was improved by the installation of partitioning plates into the riser of ELAs.