Abstract Cadmium(II), zinc(II), magnesium(II), and bis-[thallium(I)] complexes of meso-tetraphenyl-porphyrin undergo unsensitized photooxygenation to yield a bilitriene derivative as the final product. The reaction has previously been shown to involve singlet oxygen (Matsuura et al., 1980); on the basis of mass spectrometry of products from 16O2 and 18O2 experiments, and characterization of the initial photoproduct, the reaction is shown to proceed by a mechanism in which one oxygen molecule adds to the metalloporphyrin to give a metallobilitriene. In the subsequent work-up, demetalation and addition of two hydrogen atoms gives the final product.