This paper reports the results of the first detailed experimental study on probing nanoaggregation of a polyaromatic compound. Electrospray ionization mass spectrometry (ESI-MS) was used to monitor the self-association of a well-defined polyaromatic compound, N-(1-hexylhepyl)-N'-(5-carboxylicpentyl)-perylene-3,4,9,10-tetracarboxylicbisimide (CSPe), under various solution conditions. Gaseous ions corresponding to nanoaggregates of CSPe molecules were directly observed on ESI mass spectra. The dominant aggregation number (n) was found to be less than 10, although larger nanoaggregates with an aggregation number larger than 10 were also observed. The aggregation number of CSPe decreased by replacing toluene with xylene, while it increased with the CSPe concentration or upon the addition of heptane to toluene as the solvent. The consecutive aggregation number was found only for small CSPe nanoaggregates (2 <= n <= 11), which suggests a stepwise self-association at n <= 11. The larger nanoaggregates (n > 11) were formed by interactions between small nanoaggregates. The presence of naphthenic acids (NAs) was observed to hinder CSPe self-association. The dispersive effect of NAs was found to be in the order of 1-methyl-1-cyclohexanecarboxylic acid similar to cyclohexanebutyric acid < stearic acid < 5 beta-cholanic acid < 1-naphthalene pentanoic acid. The nanoaggregation behavior of CSPe was compared to that of two other polyaromatic compounds.