(Figure presented) The [2 + 2] photocycloaddition reaction of 2(5H)-furanone to ethylene and acetylene has been investigated by means of DFT and CASSCF methods. In both cases, the reaction involves the formation of a triplet 1,4-biradical intermediate that evolves to the cyclobutane product after spin inversion. For acetylene, the lowest energy path in the triplet surface occurs through the 3(π-π*) state of the 2(5H)-furanone. However, in the reaction with ethylene the lowest energy path in the triplet surface involves the 3(π-π*) state of the alkene. Although reaction through the triplet state of olefins is usually disregarded due to the short lifetime of these species, we have experimentally measured that sensitization of ethylene triplet state can occur at typical synthetic conditions and, thus, lead to photochemical addition to the lactone. © 2010 American Chemical Society.