The 1,4-cycloaddition reactions of the singlet ((1)Delta(g)) oxygen with s-cis-1,3-butadiene and benzene, with the formation of 3,6-dihydro[1,2] dioxin and 2,3-dioxabicyclo[2.2.2]octa-5,7-diene, respectively, were studied by means of the CAS MCSCF/CAS MCQDPT2 ab initio method with the 6-31G* basis set. In the case of butadiene the reaction was found to be exoenergetic and the product was found to have C-2 symmetry, with the peroxide moiety in the gauche configuration. In the case of benzene the reaction was found to be endoenergetic and the bicyclic product formed was found to have C-2v symmetry, with the peroxide moiety in the syn configuration. Three possible reaction routes were studied: (i) concerted cycloaddition, (ii) stepwise cheletropic cycloaddition with the formation of zwitterionic 2,5-dihydrofuran I-oxide as an intermediate, and (iii) stepwise cycloaddition with the formation of a linear intermediate. In the case of butadiene routes (i) and (ii) were excluded, because only second-order saddle points were found on the corresponding reaction pathways. The linear intermediate (I-1) found in route (iii) has a biradical character, and its energy relative to that of the separate reactants is 4.1 kcal/mol. The dominant activation barrier corresponds to the transition structure T-1 leading to I-1 and amounts to 9.9 kcal/mol. The rearrangement of I-1 to the product (P) involves only a minor activation barrier of 7.5 kcal/mol (relative to I-1). In the case of benzene the reaction occurs in a concerted manner with a single transition structure having C-2v symmetry; the activation barrier is 25.3 kcal/mol. This difference in binding mechanism can be explained in terms of the configuration of the peroxide moiety in the adduct.