Co-Mo/MgO-Al_2O_3 catalyst was presulfided with ammonium sulfide in aqueous solution and activated with synthesis gas for water gas shift reaction.The assay results indicate that the presulfided Co-Mo/MgO-Al_2O_3 catalyst exhibits an excellent catalytic activity and stability.XRD and EPR characterization results show that the O-S exchange might occur during the impregnation,leading to the formation of(NH_4)_2MoS_4(or(NH_4)_2Mo_xS_y)precursor,which was then thermally decomposed and reduced to MoS_2.The higher catalytic performance is attributed to an optimization formation of active Co-Mo sulfides,consisting of well dispersed MoS_2 and Co-Mo-S phase due to the redispersion of Co sulfide particles over the edges of newly formed MoS_2 crystallites.