Simultaneous hydrodesulfurization (HDS) and hydrodeoxygenation (HDO) of mercapto and/or methoxy group containing model compounds was studied on CoMo/gamma -Al2O3 in order to find out how the interactions of the substituents affect the efficiency of hydrotreating. HDS of mercaptobenzene decreased in the presence of anisole compared to the reaction of pure mercaptobenzene and a less reactive sulfur compound, (methylmercapto)benzene, was formed via methyl transfer. Simultaneously, the cleavage of the aliphatic carbon-oxygen bond in anisole was markedly enhanced due to this methyl capture by sulfur. The apparent activation energy of phenol formation from anisole decreased, indicating as well a change in the reaction mechanism. HDO reactions of anisole were, however, suppressed in the presence of mercaptobenzene. Addition of small amounts of a sulfiding agent, CS2, to the reactor feed also had a positive effect on the total conversion of anisole, but it did not markedly affect the HDO rates. With a methoxy group present in the same molecule as a mercapto group, both the HDS rate and the reaction selectivity were dependent on the isomer structure. The HDS rate of para-mercaptoanisole was about twice as high as that of mercaptobenzene at 225 degreesC, while the methoxy group in the ortho position did not markedly affect the rate. The HDS rate of meta-mercaptoanisole was about half of that of mercaptobenzene. phenol was formed from ortho-mercaptoanisole, while in other mercaptoanisole isomers the methoxy group remained predominantly unchanged. HDO reactions were suppressed in the presence of a sulfur-containing substituent but methyl transfer to. the benzene ring was not affected.