In this paper, we describe the development of a laboratory- scale delayed coker and present results of an investigation on the recovered liquid from the coking of decant oil and decant oil/ coal mixtures. Using quantitative gas chromatography/ mass spectroscopy ( GC/ MS) and H-1 and C-13 NMR, a study was made of the chemical composition of the distillate liquids isolated from the overheads collected during the coking and co- coking process. H-1 and C-13 NMR analyses of combined liquids from coking and co- coking did not show any substantial differences. These NMR results of coking and co- coking liquids agree with those of GC/ MS. In these studies, it was observed that co- coking with coal resulted in a decrease in the paraffins contents of the liquid. The percentage of cycloparaffins, indenes, naphthalenes, and tetralins did not change significantly. In contrast, alkyl benzenes and polycyclic aromatic hydrocarbons in the distillate were higher in the co- coking experiments which may have resulted from the distillation of thermally cracked coal macromolecules and the contribution of these molecules to the overall liquid composition.