A series of NiMo/gamma-Al(2)O3 carbides were synthesized with the temperature programmed carburization method for the tri-reforming of methane. The relationship between the catalytic behavior and the impregnation sequence was investigated by XRD, TG-DSC, XPS, SEM, and so on. It was found that Mo2C species were the active species for the tri-reforming of methane. The amounts of Mo2C species on the fresh NiMo carbides decreased in the order of co-impregnated NiMo carbide >Ni-first NiMo carbide > Mo-first NiMo carbide. In contrast, Mo2C species on the latter two carbides would continue to be formed during the tri-reforming of methane. NiMoO4 species only existed on the co-impregnated NiMo oxidic precursor, exhibiting a strong synergistic effect between Ni and Mo species. NiMoO4 species were reduced more easily than MoO3 species on the step-impregnated NiMo oxides. Ni species promoted the dissociation of methane, induced the reduction of molybdenum oxide and supplied active hydrogen and carbon species for the carburization process. The co-impregnation method was conducive to refine the particles. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.