Cypermethrin is a synthetic pyrethroid that has become one of the most important insecticides in wide-scale use both indoors and outdoors. Although cypermethrin is likely to become adsorbed (following its application) on aerosols and upon stagnant outdoor and indoor surfaces, its atmospheric degradation processes are not well understood yet. Here, we have quantitatively investigated the oxidation of cypermethrin by gaseous ozone, including kinetic analysis and identification of volatile and nonvolatile products. The investigation was done using a novel apparatus, combining two FTIR setups for the parallel analysis of both condensed and gas phases. The nonintrusive analysis method enabled one to follow the reaction in real time without any alteration that may affect the process. The proposed reaction mechanism involves both an ozonide formation and an ester cleavage process that occur due to secondary radical chain reactions. It was found to follow a Langmuir-Hinshelwood mechanism with a half-life time of cypermethrin in relation to average atmospheric ozone level of 50 ppb of about 21 days. These results indicate that the heterogeneous oxidation of cypermethrin by ozone cannot be neglected in the overall environmental fate cycle for this material. Furthermore, several of the yielded condensed products are toxic and more polar than the parent molecule, a fact that makes them possible contaminators of groundwater in contrast to the parent molecule. Gaseous phosgene, a known nerve gas, was found to be generated during the ozonolysis reaction, which may increase the hazard in the use of cypermethrin as an indoor insecticide.