The hydrodynamics and mass transfer characteristics of a 0.7 m(3) external loop airlift reactor were experimentally investigated for superficial gas velocities between 0.02 and 0.18 m s(-1). The reactor was made of QVF glass sections with 6.5 m in height and 0.225 m in diameter of both riser and downcomer. The effects of the addition of minute quantities of silicone polymer, polypropylene glycol and octanol, on air-water systems were examined. In general, the addition of surface active agents led to significant reductions in volumetric mass transfer coefficients and liquid circulation velocities, relative to the values determined for the air-water system. However, for the gas hold-up, gas disengagement section was found to have a great effect on the reactor performance, especially when foaming increased in the system. Of the two gas disengagement sections used (namely, closed and open channel) the closed channel configuration failed to remove the increasing small bubbles and the reactor continuity disrupted at high gas throughputs. In contrast, the open channel configuration improved the reactor performance with substantial reduction in downcomer gas hold-up and increase in volumetric mass transfer coefficient.