Two decant oils (DO A and DO B) which produce coke with different optical textures were quantitatively analyzed before and at several stages of carbonization. The differences observed in mesophase development and sulfur incorporation in the semicokes were related to differences in the molecular composition of the two decant oils, including hydrocarbons and sulfur compounds. The analysis of the GC-amenable fractions of the decant oils showed that DO A contains an abundance of pyrene and substituted pyrenes, while DO B is dominated by high concentrations of polymethylphenanthrenes. In general, DO B has a much higher proportion of alkyl-substituted PAHs and a higher concentration of n-alkanes. The presence of more reactive hydrocarbons in DO B is considered to be responsible for the inferior texture development and rapid incorporation of sulfur in the semicoke. Extensive methylation/demethylation reactions appear to occur during the early stages of carbonization; the role of these reactions in mesophase development and sulfur incorporation is addressed further in the companion papers (Energy Fuels 1997, 11, 631, 637).