Two decant oils were spiked with C-13-labeled 4-methyldibenzothiophene (4-(MDBT)-M-13) and carbonized at 500 degrees C to study the reactivity of the C-13-labeled compound. A quantitative analysis of the reaction product maltenes was performed using GC/MS. The GC/MS was also used to estimate the isotopic enrichment in C-13 of the reaction products. Data shows that demethylation and methylation of 4-MDBT are the principal reactions occurring during the early carbonization of the compound in the decant oils and that the molecular composition of the decant oils is an important factor in determining the extent of these reactions. Incorporation of 4-(MDBT)-M-13 into the solid phase (semicoke) is, therefore, attributed to an enhanced reactivity due to further alkylation of the molecule.