Abstract Ab initio molecular orbital (MO) studies of singlet oxygen reactions of olefins, enol ethers, and enamines were carried out. The relative stabilities of biradical (BR), the so-called perepoxide-like (PE), perepoxide-like zwitterionic (PEZW) and zwitterionic (ZW) intermediates for singlet oxygen reactions of these species are described theoretically, together with their ionization potentials, molecular structures and solvation energies. The activation energies for the ene and dioxetane formation reactions were found to be determined by the electron-donating properties of substituents introduced to these substrates. The following conclusions were drawn from these calculated results: (1) A nonradical transition state with a PE-like conformation is the most plausible for ene reactions of allylic olefins and enol ether with allylic hydrogens. (2) A BR mechanism is favorable for the (2 + 2) reactions of singlet oxygen with simple olefins in the gas phase. (3) A nonconcerted mechanism via polar (PEZW, ZW) intermediates is operative for (2 + 2) cycloadditions of unsymmetrical enol ethers in the solution phase, whereas the (2a + 2a) mechanism and a two-step mechanism via the PE intermediate are conceivable for the (2 + 2) reactions of symmetrical olefins in the solution phase. The implications of these theoretical results to the mechanisms of singlet oxygen reactions are discussed in relation to various experimental results, such as the stereochemistry, isotope effects, solvent effects and laser-photolysis experiments.