Unsupported and alumina-supported MoS2 and CoMoS catalysts have been compared in the hydrodeoxygenation (HDO) reaction of guaiacol (2-methoxyphenol), a typical model molecule for bio-oils coming from the pyrolysis of ligno-cellulosic biomass. The goal of this work was to understand the cobalt promoting effect on MoS2 phase in this type of catalytic reaction. It appeared clearly that in the presence of the CoMoS phase, the direct deoxygenation (DDO) pathway involved in guaiacol conversion was strongly increased as compared to the non-promoted MoS2 in the bulk or supported state. This effect is similar to the well-known increase of direct desulfurization (DDS) pathway by cobalt promoter in the hydrodesulfurization (HDS) of refractory sulfur compounds over molybdenum sulfide catalysts. (c) 2010 Elsevier B.V. All rights reserved.