On dye-sensitized photooxidation of 1-phenylcyclobutene the formation of 3- benzoylpropanal and 2-phenyl-3-hydroperoxycyclobutene was ascribed to oxidation with singlet oxygen, whereas a reactive oxygen species other than singlet oxygen was responsible for the formation of 1-(2-hydroxyphenyl) cyclopropanecarbaldehyde. Some attempts to explore the latter oxygen species were made.