3-Vinyloxetane undergoes fast thermal fragmentation as compared with oxetane or alkyloxetanes. To explain this kinetic effect, a quantum chemical investigation of the thermal decomposition of 3-vinyloxetane was carried out by the B3LYP density functional method and by the MP2 and QCISD ab initio theories using the 6-31G(d) basis set. Relative energies were evaluated on the basis of single-point QCISD(T)/cc-pVDZ energies. Computational results were properly interpreted by means of configurational analysis of wave functions and by analyzing the electronic charge density p(r) and related fields within the framework of the atoms in molecules theory. Our results predicted both the synchronous and the asynchronous concerted paths for the direct fragmentation to products. However, for the asynchronous one no biradical minimum was obtained. A two-step pathway that implies a ring expansion to 3,6-dihydro-2H-pyran was also studied. The asynchronous concerted path, assisted by an unexpected H-interaction, resulted in the most favorable reaction channel and would explain the increase of fragmentation rate of oxetanes upon symmetric substitution by a vinylic group.