An efficient synthesis of imidazoles with isotope labeling at different positions of the five-membered ring was developed. We carried out a detailed mechanistic study of the photosensitized oxidation of isotope-labeled imidazole derivatives. A new product, CO2, was observed in the photooxidation of 2-H,N1-H imidazoles, but not in 2-substitituted imidazoles. The carbon of CO2 derives from the 2C of imidazole. As shown by O-18 experiments, both oxygen atoms of CO2 originate mainly from one molecule of oxygen. Transient intermediates were detected by low-temperature NMR in the photosensitized oxidation of the isotope-labeled imidazoles. Quantitative analysis of the C-13 NMR at different temperatures and times correlates the formation of one intermediate with the loss of another, thus allowing the complete decomposition pathway of the transient intermediates to be established. Singlet oxygen reacts with 4,5-diphenylimidazole via a [4 + 2] cycloaddition to form a 2,5-endoperoxide, which, upon warming, decomposes to a hydroperoxide. The hydroperoxide in one pathway loses water to form an imidazolone 7, which is hydrolyzed to a hydroxyimidazol-2-one 11. In another pathway, the hydroperoxide rearranges to diol 8. The diol rearranges to a carbamate 9 by opening and reclosing the five-membered ring. 9 decomposes to CO2 and benzil diimine. A labile NH in the imidazole is crucial for the decomposition of the initially formed endoperoxide, otherwise the endoperoxide decomposes to regenerate starting material. Many similarities exist between the photooxidations of imidazole and guanosine in organic solvent, suggesting that the two reactions share a similar reaction mechanism with singlet oxygen.