A detailed kinetic analysis of the complex reaction systems arising from the ozonolysis of C2H4 and (CH3)(2)C=C(CH3)(2) (TME), respectively, is carried out, using master equations and statistical rate theory. The thermochemical as well as the molecular data required are obtained from CCSD(T)/TZ2P and B3LYP/DZP calculations. It is shown that the primary ozonides are not collisionally stabilized under atmospheric conditions. In the reaction sequence for O-3 + TME, the same is true for CH2=C(CH3)OOH formed from (CH3)(2)COO, which completely dissociates to give OH radicals. However, in this system, a pressure dependence is predicted for the relative branching fractions of the reactions of the Criegee intermediate. Under atmospheric conditions, for both examples, the product yields obtained are in reasonable agreement with experimental results.