Recent studies indicate that secondary ozonides of cyclic alkenes are formed in atmospheric reactions and may be relatively stable. The secondary ozonides (SOZs) of cyclohexene (1), 1-methylcyclohexene (2), 4-isopropyl-1-methylcyclohexene (3) and 4-isopropenyl-1-methylcyclohexene (limonene) (4) have been characterized by rapid gas chromatography electron ionization (EI), negative and positive chemical ionization (CI: ammonia, isobutane and methane) and collision-induced dissociation (CID) mass spectrometry. Both EI and positive CI spectra were found unsuitable for reproducible analysis. However, negative CI showed stable (M-H)(-) ions with minor fragmentation. CID of the (M-H)(-) ions resulted in simple and reproducible fragmentation patterns for all four SOZs with loss of m/z 18, 44 and 60, tentatively assigned as H(2)O, CO(2) and C(2)H(4)O(2) or CO(3), respectively. Thus, negative CI-MS-MS in combination with rapid gas chromatography is the preferred method for identification of secondary ozonides of cyclohexenes.