The main purpose of this study is to assess the relative importance of diradical or peroxirane (perepoxide) intermediates in the singlet oxygen cycloaddition reactions with alkenes that lead to dioxetanes. The relevant nonconcerted pathways are explored for ethene, methyl vinyl ether, and s-trans-butadiene by CAS-MCSCF optimizations followed by multireference perturbative CAS-PT2 energy calculations and by DFT(B3LYP) optimizations. The two different theoretical approaches gave similar results (reported below). These results show that methoxy or vinyl substitution does not affect qualitatively the reaction features evidenced by the unsubstituted system. Peroxirane turns out to be attainable only by passing through the diradical, due to the nature of the critical points involved. The energy barriers for the transformation of the diradical to peroxirane in the case of ethene (Delta E-not subset of = 13-15 kcal mol(-1)) and methyl vinyl ether (Delta E-not subset of = 12-13 kcal mol(-1)) are higher than those for the diradical closure to dioxetane (Delta E-not subset of = 8-9 kcal mol(-1), for ethene, and 9 kcal mol(-1) for methyl vinyl ether). In all three systems, the peroxirane pathway to dioxetane is prevented by the high energy barrier for the second step, leading from peroxirane to dioxetane (Delta E-not subset of = 26-27, 27-31 and 22 kcal mol(-1), for ethene, methyl vinyl ether, and butadiene, respectively). By contrast, peroxirane can very easily back-transform to the diradical (with a Delta E-not subset of estimate of 3 kcal mol(-1), for ethene and methyl vinyl ether, and close to zero, for butadiene). These results indicate that, although a peroxirane intermediate might form in some cases, it corresponds to a dead-end pathway which cannot lead to dioxetane.