The [3+2] cycloaddition (32CA) reaction of an azomethine ylide with methyl vinyl ketone in the ground and first triplet excited states has been studied within the framework of molecular electron density theory. A density functional theory-based reactivity analysis indicates that, while azomethine ylide behaves as a supernucleophile in both states, vinyl ketone acts as a strong electrophile. This 32CA reaction presents a very low activation energy both in the ground state, 3.40 kcal mol-1, and in the triplet state, 1.47 kcal mol-1, in the gas phase. While in the ground state, the 32CA reaction is entirely ortho regio and fully endo stereoselective, in the triplet state it is entirely ortho regioselective but only partially endo stereoselective. In the ground state, the reaction proceeds to form the spirooxindole after passing through the transition state structure, whereas in the triplet state, the reaction stops at a biradical intermediate, which requires an intersystem crossing to yield the final spirooxindole. Analysis of the kinetic parameters of the 32CA reaction in the two states in methanol indicates that in the triplet state, the reaction is only 589 times faster than in the ground state.