Mesophase development during the carbonization of needle coke feedstocks in delayed coking affects the microstructural anisotropy (or graphitizability) of the resulting coke. Previous work has employed gas chromatography/mass spectrometry (GC/MS) to study the relationships between the molecular composition of the fluid catalytic cracking decant oil (FCC DO) and mesophase development. In this study, the principal emphasis was placed on analyzing the non-GC amenable fraction of DO samples using a combination of analytical techniques, including high pressure liquid chromatography with photodiode array (PDA) and mass spectrometer detectors in tandem (HPLC/PDA and HPLC/MS/MS), and laser desorption mass spectrometry (LD/MS). The experimental data showed that major molecular species in DO samples consist of 3-6 ring multimethyl-substituted polyarornatic hydrocarbons (PAHs), including phenanthrenes, pyrenes, chrysenes, benzopyrenes, perylenes, and benzo[g,h,i]perylenes, Large differences were observed in the molecular composition of decant oil samples, particularly in the distribution of the methylPAH homologues and the degree of methylation on a given PAH. High-boiling PAHs (identified by LD/MS) and low-boiling PAHs (identified by GUMS) were found to have similar distribution trends in molecular composition. The degree of mesophase development was found to depend strongly on the molecular composition of the feedstock samples. In particular, the ratio of total concentrations of pyrenes/phenanthrenes correlates with the degree of mesophase development during carbonization.