In the delayed coking process, the actual feed to the coking drum consists of a high-boiling fraction of the feedstock, fluid catalytic cracking decant oil (FCC-DO), and the high-boiling end of the liquid products produced by delayed coking (recycle). As distinct from the parent decant oil feed, the molecular composition and coking behavior of actual feed samples to the coking drum (CF) are reported for the first time in this paper. This study examines a commercial pretreatment approach where the feedstock to the fractionator column includes a hydrotreated fraction (HYD) and a vacuum tower bottom (VTB) fraction of a decant oil. Samples of two sets of decant oils including the corresponding HYD, VTB, and CF derivatives were analyzed and carbonized in laboratory reactors to monitor mesophase development from these materials. Decant oil samples and their derivatives, HYD, VTB, and CF, have substantially different molecular composition. The CF samples are characterized by lower degrees of methyl substitution on polyaromatic hydrocarbons (PAH) with higher proportions of thermally stable methyl-PAH isomers. A higher proportion of hydroaromatics and lower concentrations sulfur-containing aromatics characterize the HYD samples, white VTB samples consist exclusively of aromatic ring systems with greater than three condensed rings. Significant differences were also found in the mesophase development from the decant oils and their derivatives. The CF, HYD, and VTB samples produced higher degrees of mesophase development than that obtained from the parent DO.