Conventionally, the performance of a UV reactor is evaluated using the concentration of photoreactive chemicals at the outlet vs. the inlet. This research presents a novel method for measuring the concentration distribution of a photoreactive chemical inside a photoreactor using a modified planar laser-induced fluorescence method. The fluence distribution was measured for a pilot scale photoreactor under different operating conditions. The visualized result of the fluence distribution revealed significant information about the local/overall performance of the photoreactor. This method is a powerful diagnostic tool for the determination of the local performance inside a UV reactor, as well as for the evaluation of models simulating UV reactor behavior. (C) 2010 American Institute of Chemical Engineers AIChE J, 57: 208-217, 2011