Abstract—The rate constantk5/>for physical quenching of singlet oxygen O2(δ1;) by the sensitizer in dye‐sensitized photooxygenations is determined in the case of chlorophyllsaandb(7.3 times 108, 4.2 times 108M‐1s‐1respectively), pheophytinsaandb(7.4 times 107, 3.0 times 107M‐1s_1respectively), tetraphenylporphyrin (4.4 times 107M‐1s_1), magnesium tetraphenylporphyrin (5.0 times 108M‐1s_1), zinc tetraphenylporphyrin (1.5 times 108M‐1s_l) and protoporphyrin IX‐dimethylester (9.1 times 107M‐1s_1) in benzene. These sensitizers show a linear correlation between logksO, and their half‐wave oxidation potentials and the value of the slope is similar to that observed for various compounds such as phenols. It is concluded that (i) the interaction between chlorophylls and related compounds with singlet oxygen may involve an exciplex as for phenols, and (ii) physical quenching may be envisaged as a spin‐orbit‐induced intersystem cr