Microreactors receive a lot of interest in photochemical application, however, the fundamental effects of micro flow on the photochemical reactions have not been fully understood so far. We report on the investigation of the combined effect of temperature and concentration of dissolved oxygen on degradation of 4-chlorophenol in photo microreactor. Sulfonated zinc phthalocyanine was used as a photosensitizer to generate singlet oxygen. Temperature and dissolved oxygen affect the overall reaction, however, the effect is not straightforward. To investigate their combined effect, the experiments were carried out in the advanced opto-chemical apparatus with a thin-gap photo microreactor. To obtain accurate information on actual process conditions, temperature of the reaction zone was measured by an integrated temperature sensor and the concentration of dissolved oxygen was measured by the in-line oxygen probe. As results, the reaction rate constant of the model reaction was evaluated and the apparent activation energy was calculated. (C) 2015 Elsevier B.V. All rights reserved.