Nonvolatile hydrocarbon systems, such as those formed via molecular weight growth reactions in refinery processes, normally have a low H/C atomic ratios (<0.5) and high boiling/decomposition points (> 600 degreesC). These materials pose a great challenge to mass spectrometric analysis due to difficulties in vaporization and ionization. In this work, the combination of In-Source Pyrolysis Mass Spectrometry (ISPyMS) and Field Desorption Mass Spectrometry (FDMS) were utilized to obtain compositional information for a nonvolatile organic deposit (foulant) generated by a hydrocracking process. A mixture of polystyrene standards was also analyzed for reference purposes. FDMS yields intact molecular ions for the large polyaromatic systems (similar to 15 aromatic rings on average). ISPyMS, on the other hand, converts the large multi-core aromatic systems into relatively small single aromatic cores that could be analyzed in detail. In this case, 28 aromatic cores containing short alkyl chains were identified by ISPyMS. Average foulant composition was projected on the basis of IsPyMS and FDMS data. The projected WC atomic ratio and aliphatic carbon/aromatic ratio match well with those measured by elemental analysis and solid-state C-13 NMR.