Nickel (Ni)/cobalt (Co) promoted MoS2 catalysts have been recently studied in deoxygenation of vegetable oils. Ni and Co play a key role in enhancing the catalytic activities and selectivity of MoS2. To better understand their functions, nanosized unsupported NiMoS and CoMoS catalysts were studied in this work. Without the influence of catalyst supports, the inherent impact of nickel and cobalt on the main deoxygenation routes has been investigated. Ni-promoted catalyst exhibits higher selectivity for hydrodeoxygenation (HDO) reaction over hydrodecarbonylation/decarboxylation (HOC), while Co-promoted catalyst prefers HDC. Based on the analysis of upgraded products and catalysts, a new insight was proposed to interpret the impact of catalyst structure on deoxygenation pathways. NiMoS creates abundant sulfur vacancies that improve the hydrogenation ability; CoMoS catalyst shows saturated edge sites in hydrogen atmosphere, facilitating the hydrocracking (C C hydrogenolysis). The enhancement of HDO reaction by Ni is primarily realized through the synergy effect of Mo and Ni on unsaturated sites of metal edge. By contrast, the HOC reaction is facilitated by Co-promoted MoS2 through adsorption of C atoms on the S edge that is adjacent to Co. (C) 2014 Elsevier B.V. All rights reserved.