A photo microreactor (PMR) of unique parameters was operated in the degradation of 4-chlorophenol with sulfonated zinc phthalocyanine (ZnPcSmix). This sensitizer was chosen as model homogeneous photocatalyst to evaluate operation parameters and efficiency of the PMR. All reactions were carried out in deuterium oxide to maximize the life-time of the generated singlet oxygen species. In parallel same model oxidations were performed in a laboratory batch system using identical optical arrangements. Both systems were optimized and evaluated with help of kinetic models and rationally compared. It was shown the PMR arrangement revealed much higher process efficacy. Construction details and optimized operating parameters of the PMR are provided and discussed in details. (C) 2014 Elsevier B.V. All rights reserved.