Quantum chemical calculations are performed to explore all of the possible pathways for primary ozonide (POZ) formation from gas-phase ozonolysis of catechol. Canonical transition state theory has been used to calculate the rate coefficients of individual steps for the formation of POZ. The calculated rate coefficients for 1,3-cycloaddition of ozone at the (i) unsaturated C(OH)═C(OH) bond and (ii) CH═C(OH) of catechol, respectively, are in good agreement with the experimental rate constant. In general, subsequent decomposition of POZ leads to well-known Criegee Intermediates. This work reveals a parallel pathway by which the endo-addition of ozone at CH═C(OH) of catechol proceeds through oxepin derivatives along with the paths leading to Criegee Intermediates and peroxy acids. The 7-membered heterocyclic oxepin derivatives have lower energies than Criegee Intermediates but similar relative energies with peroxy acids.