BACKGROUND: There is scarce information on the application of electrical resistance tomography (ERT) in UV photoreactors, in which mixing and mass transfer are important. Therefore, the feasibility of an ERT system in an agitated multi-lamp UV photoreactor was investigated to monitor the mixing process. RESULTS: The locations of the UV tubes had a significant impact on the mixing time, particularly at the lower impeller speeds (45 and 150 rpm). Also, at the higher impeller speeds (250, 350, and 500 rpm) and the same radial position (r), changing the angle theta from 15 to 45 degrees, resulted in only a slight variation of the mixing time. Finally, the maximum mixing time occurred when UV tubes were positioned at r = 13 cm (r/R = 0.68) and theta = 0 degrees, while minimum mixing time occurred at location r = 16 cm (r/R = 0.83) and theta = 45 degrees. CONCLUSION: The experimental results demonstrated the feasibility of the ERT system to monitor the mixing process in the UV photoreactor. The ERT results also indicated that the locations of the UV tubes had a significant effect on the mixing performance of the photoreactor. Furthermore, the mixing time varied inversely with the rotational speed, and this effect was more pronounced at lower speeds. (C) 2008 Society of Chemical Industry