Reactions of cyclic alkenes and O-3 are known to form significant quantities of aerosol, but the chemical mechanism and complete identity of the products has not been established. In this study, a thermal desorption particle beam mass spectrometer is used to investigate the ozonolysis of cyclohexene and some of its structural homologues, including cyclopentene, cycloheptene, and cyclooctene, in humid air. Dicarboxylic acids, which are the lowest volatility compounds previously identified in aerosol formed from this reaction, are also observed here. But in addition, the thermal desorption behavior of the aerosol indicates that compounds less volatile than dicarboxylic acids are present. On the basis of mass spectra and desorption behavior of the aerosol and standard compounds, and chemical probe experiments, it is proposed that these compounds are diacyl peroxides. A reaction mechanism is presented to explain the formation of diacyl peroxides, as well as dicarboxylic acids. These results may have important implications for understanding the potential role of organic compounds in nucleation and growth of atmospheric aerosol particles.