The ground and first triplet excited-state potential energy surfaces of the [2 + 2]-cycloadditions of 2-cyclohexenone, methyl acrylate, and methyl crotonate to ethylene have been studied by means of CASSCF and DFT-B3LYP calculations. The attack of ethylene to the (3)(pi-pi) alpha,beta-unsaturated carbonyl compound leads to the formation of a triplet 1,4-biradical intermediate that evolves to the ground-state potential energy surface. The outcome of the reaction is governed by the competition between the deactivation of the (3)(pi-pi) alpha,beta-unsaturated carbonyl compound itself and its reaction with ethylene to form the triplet 1,4-biradical. For 2-cyclohexenone, the potential energy barrier corresponding to the formation of the biradical intermediate is lower than for the acyclic systems. On the other hand, the energy necessary to reach the crossing point between the (3)(pi-pi) and the ground-state potential energy surfaces is lower for the acyclic systems than for 2-cyclohexenone. For methyl acrylate and methyl crotonate, the decay of the (3)(pi-pi) state of the isolated molecule is therefore expected to be faster than the formation of the 1,4-biradical, so that the [2 + 2]-cycloaddition will not take place. However, for 2-cyclohexenone the formation of the triplet 1,4-biradical is favorable, and the process will lead to the formation of the corresponding cyclobutane derivative.