Local mass transfer studies have been carried out in an external loop airlift reactor and indicated the existence of significant non-uniformities in aeration capacity in the various sections of the reactor. The traditional assumption of a single well mixed unit with a single k(L) a value for external loop airlift reactors was found to be poor and cannot represent adequately the reactor as a whole. The liquid head in the gas/liquid separator was found to have a significant negative effect on the local mass transfer coefficient k(L) a. Visualization of flow patterns showed the existence of strong vortices in the riser and downcomer which contributed to a reduction in local aeration capacity. Mass transfer experiments in an aerated tank showed that the aeration capacity can be reduced by half in viscous media but improved significantly in the presence of salts.