A low cost optical trajectory tracking system (OTTS) for flow studies has been developed in our laboratory. The system was developed with the purpose of identifying flow trajectories, which are required for the mathematical formulation of photosynthetic processes in airlift reactors. It consists of two cameras, which are connected to a PC computer via,an Image Process Card. The system is used for the study of fluid trajectories in the downcomer of an airlift reactor. The motion of a single neutrally buoyant fluorescent particle is logged by the system, and the vectors of the trajectories are reconstructed into 3D coordinates. The flow in the downcomer of the airlift reactor was studied using two different draft tube diameters. The superficial gas velocities varied from 0.005 to 0.025 m/s. Analysis of the experimental results shows that plug flow exists in this region. Shear stress was also analyzed and was found homogeneous. Preliminary results of the measurement of fluid flow in the downcomer of the ALR with helical flow promoters (HFP) are also presented. (C) 2003 Elsevier Science Ltd. All rights reserved.