This study presents the first combined application of C-13 labeling and isotope-ratio-monitoring gas chromatography-mass spectrometry (irmGCMS) to analyze the thermochemical transformations of specific organic compounds within a complex heavy oil. Specifically, this technique was used to monitor the hydrocarbons produced during the carbonization of a fluid catalytic cracking decant oil (FCCDO). Structural GC/MS analysis of these products has previously shown that 4-methyldibenzothiophene, C-13-labeled at the methyl position (4-(MDBT)-M-13), undergo es methylation to form a number of polymethyldibenzothiophenes and cleavage of its aryl-methyl bond to give dibenzothiophene (Energy Fuels 1997, 11, 623, 631). The irmGCMS analysis of these products shows that the 4-(MDBT)-M-13 is a source of C-13-enriched methyl that reacts with polyaromatic hydrocarbons (PAH) to produce a number of methylated PAHs. Precise measurement of C-13 enrichment and concentration of specific compounds enables tracking of the labeled carbon within a complex mixture as a function of reaction time. There are large differences in apparent reactivities of mono- and dimethyl-substituted naphthalenes, phenanthrenes, and pyrenes. The differences in observed reactivities of methylnaphthalene, methylphenanthrene, and methylpyrene isomers are consistent with calculated free valence indices at specific positions. There is no significant C-13 enrichment in unsubstituted PAH compounds, indicating that the labeled methyl group is not involved in producing GC-amenable unsubstituted PAH during early carbonization. This combined use of C-13-labeled reactants and irmGCMS analysis will have broad applications in oil processing and geochemical fields associated with the study of thermochemical transformations of organic compounds within complex mixtures.