Interaction of sulfur model compounds with aliphatic, aromatic and hydroaromatic hydrocarbons under various conditions mimicking proposed coprocessing techniques are described. Results suggest that aliphatic thioether compounds will be much more reactive than aromatic sulfur compounds unless an effective hydrogen supply is present to hydrogenate these compound types. In the absence of an effective hydrogen source, phenyl sulfide and benzothiophene underwent retrogressive reactions producing higher molecular weight products containing new aromatic C-C and C-S bonds.