The reaction mechanism of the [3 + 2] intramolecular cycloaddition of 3,3-dimethyl-2-(prop-2-en-1-yloxy) and (prop-2-en-1-ylsulfanyl) nitrile oxides is analyzed using different DFT functionals with the 6-311++G(d,p) basis set. The activation and the reaction energies for the cis and trans pathways are evaluated at the DFT, MP2, and CCSD(T) levels of theory as well as their Gibbs free energy counterparts. It is shown that the trans regioisomers are both thermodynamic and kinetic compounds, in agreement with experimental outcomes. For a deeper understanding of the reaction mechanism, a BET analysis along the reaction channel (trans and cis) has been carried out. This analysis reveals that the lone pair on the nitrogen atom is formed first, then the C-C bond, and finally the O-C one. The global mechanism is similar for the two compounds and for the two pathways even if some small differences are observed, for instance, in the values of the reaction coordinates of appeareance of the different basins.