The reactions of O 2 (-.) with alkyl and aryl esters of benzenedicarboxylic acids have been studied under negative-ion chemical ionization (NICI) conditions via a conventional chemical ionization source. Reaction mechanisms have been elucidated by using ion isolation techniques on a Fourier transform ion cyclotron resonance mass spectrometer. In addition, (18)O 2 (-.) has been used as the reagent and the products of competitive reactions that involve the mixed esters of benzenedicarboxylic acids have been studied. O 2 (-.) reactions with the alkyl esters of 1,2- and l,3-benzenedicarboxylic acids are attributed to SN2 displacement at the O-alkyl carbon. The spectra of mixed alkyl esters show that O 2 (-.) attack is reduced at sterically hindered alkyl groups. In contrast with the spectra of 1,2- and l,3-benzenedicarboxylic acids, the spectra of 1,4-benzenedicarboxylic acids are dominated by M(-.) production. Reactions of O 2 (-.) with phenyl benzoates and the aryl esters of benzenedicarboxylic acids proceed via addition-elimination pathways. Experiments with mixed alkyl-aryl benzenedicarboxylic acid esters show that the addition-elimination reaction pathway is preferred over O-alkyl SN2 displacement. The O2/ Ar-NICI mass spectra show features that can be used to distinguish 1,2-, 1,3-, and 1/4-benzenedicarboxylic acid esters. Molecular and fragment ions provide structural information complementary to that generated under electron ionization and chemical ionization conditions.