When gases flow through an annular photoreactor at constant rate, some of the gas spends more or less than the average residence time in the reactor. This spread of residence time can have an important effect on the performance of the reactor. This study tested how the residence time distribution (RTD) in an annular photoreactor with a magnetic stirrer at the bottom, is affected by the flow rate, different stirring rates and reactor length-to-diameter ratios. Pulse response studies with UV-Vis measurements were used to measure RTD curves, and a dimensionless parameter L/[N(D-0 - D-i)] was used as a measure of the approach to plug flow. Numerical method was used to develop a model base on second-order discritization and a convergence criteria of 10(-4) for all variables as laminar flow. Effects of reactor mixers in an annular photoreactor were simulated using a finite volume method (Fluent). Steady state solutions were obtained by imposing boundary conditions of inlet velocity of the required flow rate, inside and outside surface of the cylinder specified a wall boundary condition and outflow boundary conditions adapted at the outlet boundary. A virtual fan having specific radial and axial flow velocities introduced at the inlet of the reactor to initiate swirl motion. Grids for the above simulations are generated using Gambit. Qualitative comparisons of the numerical results with experimental results showed that the use of axial or mixed flow stirrer could improve the flow profile narrowing the RTD curve, creating high Reynolds numbers and avoiding back mixing. The information would be useful to design and scale-up gas flow photoreactor that behave like stirred tanks in series or approach plug flow system. (C) 2003 Elsevier B.V. All rights reserved.