This study reports an experimental investigation into the hydrodynamic behavior of an external-loop airlift reactor (ALR) for the air-water system. Three distinct flow regimes are identified-namely homogeneous, transition, and heterogeneous regimes. The transition between homogeneous and heterogeneous flow is observed to occur over a wide range rather than being merely a single point as has been previously reported in the literature. A gas holdup correlation is developed for each flow regime. The correlations fit the experimental gas holdup data with very good accuracy (within +/-5%). It would appear that a deterministic equation to describe each flow regime is likely to exist in ALRs. This equation is a function of the reactor geometry and the system＇s physical properties. New data concerning the axial variation of gas holdup is reported in which a minimum value is observed. This phenomenon is discussed and an explanation offered. Discrimination between two sound theoretical models-namely model I (Chisti et al., 1988) and model II (Garcia Calvo, 1989)-shows that model I predicts satisfactorily the liquid circulation velocity with an error of less than +/-10%. The good predictive features of model I may be due to the fact that it allows for a significant energy dissipation by wakes behind bubbles. Model I is now further improved by the new gas holdup correlations which are derived for the three different flow regimes.