On the basis of spin-polarized periodic density functional theory including the latest dispersion correction (PBE-D3), the mechanisms of H-2, O-2, H2O, CO2, CO, CH4 and C2Hx dissociative adsorption on the hexagonal Mo2C surface have been computed. In our study we used the metallic Mo2C(001) surface as well as the Mo2C(101) surface with Mo/C = 1/1 ratio. It is found that the dissociative adsorptions of these small molecules are exothermic and have low barriers; and the Mo2C(001) surface has much stronger dissociative adsorption than the Mo2C(101) surface. In contrast to the Mo2C(001) surface, OH+H and O+2H can form equilibrium on Mo2C(101) surface. For C2Hx dissociative adsorption on the Mo2C(001) surface, C-H bond dissociation is kinetically much more favorable than the C-C bond dissociation, and the optimum C2H6 dissociation route follows the order of C2H6 -> CH3CH2 + H -> C2H4 + 2H -> CH2CH + 3H -> C2H2 + 4H -> C2H + 5H -> C-2 + 6H. Due to the very strong dissociative adsorption energies, both surfaces can be oxidized easily by using H2O; and high oxygen coverage can be expected. The Mo2C(001) surface can uptake more surface O atoms than the Mo2C(101) surface. These surfaces can also be carburized by using CH4, albeit in less extent. On the surface with co-adsorbed CO2 + 4H; CO2 dissociation (CO2 -> CO + O -> C + 2O) is more favorable than the hydrogenation of CO2 (CO2 + H -> HCOO or COOH) and CO (CO + H -> HCO or COH). It is noted that CO2 hydrogenation towards CH4 formation is unlikely on the Mo2C(001) surface, while the effective barrier of surface C hydrogenation on the Mo2C(101) surface can be reduced by 2O and 2OH pre-covered surfaces. (C) 2016 Elsevier B.V. All rights reserved.