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Asplit-cylinder airlift reactor was used to characterize the effects of Newtonian viscosity on the induced liquid circulation rate, the overall gas holdup, the distribution of holdup between the riser and the downcomer zones, and the mixing time. Over the viscosity range (1.54–19.50)×10⁻³ Pa s, the viscosity did not affect the liquid circulation rate because any viscosity-associated increase in frictional resistance to flow was compensated by a 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 …