This paper describes a series of experiments undertaken to evaluate the influence of liquid viscosities of up to 17 mPa s on the hydrodynamic behaviour of an external-loop TPAL reactor. Mixtures of glycerol and water were used as model viscous Newtonian fluids. The primary effects of the increase in viscosity were to reduce the terminal settling velocity of the solids, and to increase the system head losses. Empirical expressions describing the gas holdups in the riser and downcomer were obtained from the data. These have been used to generate solutions for a mathematical model describing the system, which enable the prediction of the reactor stalling point and the magnitude of the liquid recirculation velocity (U-LR) for a given gas flowrate. Experimental measurements of U-LR have been compared with predicted U-LR values obtained using the mathematical model, and there is good agreement between the two.