For the first time, the application of microreactors as a tool for acquiring kinetic data on a photochemical reaction is demonstrated. For illustration, a T-photochromic system is considered. By using modeling tools and carrying out specific experiments in a spiral-shaped microreactor irradiated by an ultraviolet/light-emitting diode (UV-LED) array, the two kinetic parameters of the reaction, namely, quantum yield and rate of thermal back reaction, are determined. Once these parameters are known, the photochromic reaction is performed in two other microreactors in order to investigate a wider range of operating conditions. It is observed that a critical residence time exists beyond which the conversion into the open form decreases due to a decomposition reaction. The value of the critical residence depends on the microreactor type, which can be predicted by applying the model developed.