Abstract— A kinetic investigation was performed on the photooxidation of methionine sensitized by various porphyrins at different oxygen concentrations. The rate of photooxidation was found to be strongly dependent on the nature of the sensitizer. In the case of hematoporphyrin, chelation of Mg2+ and Zn2+ and especially of Cu2+ and Fe2+ caused a significant decrease of the photosensitizing efficiency. Fluorescence and/or flash photolysis studies showed that such a decrease is ascribed to an enhancement of the non‐radiative decay of the first excited singlet state as well as to a reduction of the triplet lifetime. The sensitizing efficiency is also dependent on the nature of the porphyrin side chains. A reaction mechanism involving 1O2 as the oxidizing agent is proposed.