Partially oxidized derivatives of polycyclic aromatic hydrocarbons (PAHs) are known to be important environmental pollutants. For the identification of these substances in complex mixtures, e.g. atmospheric aerosol samples, liquid chromatography/mass spectrometry with atmospheric pressure chemical ionization (LC/APCI-MS) has been found to be a suitable analytical technique. In this study 31 derivatives of mono- and polycyclic aromatic hydrocarbons with up to five condensed aromatic rings carrying different functional groups (carboxyl, dicarboxylic anhydride, lactone, hydroxyl, and carbonyl) were characterized by LC/APCI-MS. Each substance was measured in positive and negative ion detection mode at four different fragmentor voltages (90 to 190 V). For the first time, the results show that characteristic and well-interpretable fragmentation patterns can be obtained for these classes of compounds by in-source collision-induced dissociation in a single quadrupole LC/APCI-MS system. For each class of compounds typical spectral features and optimum measurement conditions are reported, and fragmentation pathways are proposed. The study demonstrates the applicability of LC/APCI-MS for the determination of most of the investigated compounds at trace levels, and it provides a database for the identification of unknown partially oxidized aromatic hydrocarbons. Copyright 1999 John Wiley & Sons, Ltd.