Hydrodynamics in the downcomer of a high Liquid holdup airlift tower loop reactor, whose riser has been previously studied, is presented. A visual procedure with colorimetric tracers injected into the top part of the riser and monitored by means of a video camera system, for subsequent treatment of the obtained images, was used. The tracer front advance curves have suggested a compartmental model of three parameters to be used for physical transport processes.