The effect of mechanical-chemical activation (MCA) of Ni(Co)-Mo catalysts, supported on gamma-Al2O3 has been studied with respect to their phase composition, structure, catalytic activity and selectivity in the reaction of thiophene hydrodesulphurization (HDS). The mechanical-chemical treatment with a duration of 7 h has been accomplished with a planet-ball mill in air medium. The samples have been characterized by means of XRD, XPS, SEM and TPR prior to and after the MCA. The specific surface area and pore size distribution curves have been determined by recording of N-2 adsorption isotherms using BET method. The obtained results show that the HDS activity of the mechano-chemically-activated catalysts is considerably higher than their activity prior to this type of treatment. The applied mechanical-chemical activation caused a partial amorphization of the active components, deposited on the support, a decrease in the average particle size, separation and partial migration of the supported phase. The activation led to redistribution of the Mo6+ ions on the surface, whereupon the share of the Mo6+ ions, involved in the formation of the superficial Ni(Co)-Mo-O phase, became larger. (c) 2005 Elsevier B.V. All rights reserved.