An annular flow photoreactor for the direct photolysis of 2,4-dichlorophenoxyacetic acid has been developed, mathematically modeled and experimentally verified in a bench scale apparatus. The model employs a very simple kinetic equation that was previously obtained in a well stirred tank, batch laboratory reactor. Reasonably good agreement has been obtained between model predictions and experimental results. The observed errors are mainly due to the fact that the kinetics of this very complex reaction have been modeled in terms of just one single concentration.