A mechanism for the reaction of the NO3 radical with propene is proposed on the basis of B3LYP and CASSCF; calculations. The mechanism involves initially both Markownikoff and contra-Markownikoff oriented addition on the double bond. From the initial radical adducts several interconnected pathways have been found, giving 1,2-epoxypropane, propanal, propenol, propanone, formaldehyde, ethanal, NO, and NO2. The geometries of the stationary points found on the potential energy hypersurface (PES) were obtained and characterized by means of density functional theory (DFT) B3LYP/6-31G* calculations. Comparison of the optimized geometries on a limited number of molecular structures was carried out with calculations at the CASSCF level of theory with the same basis set, building the active space with five electrons in six orbitals.