Photoionization mass spectrometry (PIMS) has been used to study the dissociative ionization of three anthropogenic atmospheric aerosol precursors (o-xylene, 2-methylstyrene, indene) and five of their main atmospheric degradation products (o-tolualdehyde, 2-methylphenol, o-toluic acid, phthalic acid, and phthaldialdehyde). Ionization and fragment appearance energies have been experimentally determined in the 7-15 eV photon energy regime. Moreover, intensive ab inito quantum chemical calculations have been performed to compute the first ionization energies and heats of formation of these compounds (including also phthalic anhydride). Several methods have been used, and the theoretical results are compared to the experimental values with the aim to find the best method to predict thermochemical data for similar molecules. The vacuum-UV fragmentation pathways following photoionization are discussed. The results of this work are important with respect to the analytical chemistry of these compounds since their basic gas phase ion energetics data are mostly unknown. They will help in interpreting real-time mass spectrometric measurements used for the study of organic aerosol formation in smog chambers and in the real atmosphere.