The O-3-initiated oxidation reactions of a-pinene ([1S,5S]-2,6,6-trimethylbicyclo[3.1.1]hept-2-ene), beta-pinene ([1R,5R]-6,6-dimethyl-2-methylenebicyclo[3.1.1]heptane), camphene ([1R,4S]-2,2-dimethyl-3-methylenebicyclo[2.2.1]heptane) and sabinene ([1R,5R]-4-methylene-1-(1-methylethyl)bicycle[3.1.0]hexane), four monoterpenes typically emitted into the atmosphere, were studied at the B3LYP/6-31+G(2d,2p) level of theory. The rate coefficients were calculated on the basis of the variational transition state theory for two kinetic models, in order to investigate the reaction mechanism: first assuming a direct bimolecular reaction and the second, assuming the formation of a prebarrier-complex, which further reacts forming the corresponding molozonide. The barrier heights leading to the formation of exo-conformers of the molozonides of alpha-pinene and camphene are lower than the barrier heights for the formation of the endo-conformers of these molozonides, whereas the inverse trend is observed for beta-pinene and sabinene. The canonical variational rate coefficients are found in reasonable agreement with the experimental data, especially when the prebarrier complexes are considered. Microcanonical variational rate coefficients are also calculated, as a final validation test, being found in an expected agreement with the canonical rate coefficients. The best predictions for the rate coefficients at 298 K, based on the microcanonical variational method, for a-pinene, beta-pinene, camphene, and sabine are (in units cm(3) molecule(-1) s(-1)): 6.92 X 10(-17), 1.06 X 10(-17), 4.61 X 10(-19), and 4.81 X 10(-17), respectively. Our results suggest that the prebarrier complex is an important specie in the ozone addition mechanism and should be taken into account for the proper description of the overall kinetics.