corresponding increase in the driving force for liquid circulation. The overall gas holdup declined with increasing viscosity, but the difference between the gas holdups in the riser and the downcomer zones increased. The mixing time was little affected by the viscosity. The observed mixing behaviour was satisfactorily correlated by a mechanistic equation that associated mixing to relative velocity between the phases and the length of the circulation loop. The ratio of the mixing time and the liquid circulation time was constant irrespective of the aeration rate or the viscosity of the liquid.