A laboratory study was carried out to investigate the chemical composition of aged aromatic secondary organic aerosol (SOA) formed from the photoxidation of p-xylene in the presence of ammonia (NH3). The experiments were conducted by irradiating p-xylene/CH3ONO/NH3 air mixtures without and with NO in a home-made smog chamber. The particulate products of aged p- xylene SOA in the presence of NH3 were measured by UV-vis spectrophotometry, attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy, and aerosol laser time-of-flight mass spectrometry (ALTOFMS) coupled with the fuzzy C-means (FCM) clustering algorithm. The experimental results show that NH3 does not alter the gas-particle partitioning in the photoxidation of p-xylene without NO and that 2,5-dimethylphenol is the predominant NH3-aged p-xylene SOA without NO. However, NH3 has a significant promotional effect on the formation of organonitrogen compounds in the OH-initiated oxidation of p-xylene with NO. Organic ammonium salts such as ammonium glyoxylate and p-methyl ammonium benzoate, which are formed from NH3 reactions with gaseous organic acids, were detected as the major particulate organonitrogen products of NH3-aged p-xylene SOA with NO. 1H-Imidazole, 4-methyl-1H-imidazole, and other imidazole products of the heterogeneous reactions between NH3 and dialdehydes of p-xylene SOA were newly measured. The possible reaction mechanisms leading to these organonitrogen products are also discussed and proposed. The formation of imidazole products suggests that some ambient particles containing organonitrogen compounds may be the result of this mechanism. The results of this study may provide valuable information for discussing anthropogenic SOA aging mechanisms.