A comparative study has been made on the efficiency of leaching, using different concentrations of oxidized oxalic acid, and thermally treating techniques to reactivate a Ni-Mo-spent hydrotreating catalyst. The reactivation of the treated catalysts was performed in a pulse microcatalytic reactor under atmospheric pressure of hydrogen and reaction temperatures ranging from 250 degrees C to 450 degrees C using cyclohexene as model hydrocarbon. The studies revealed that a decoking spent catalyst after leaching using a low concentration (2%) enhanced its physical characteristics (surface area and average pore diameter) and catalytic activity towards maximum cyclohexene hydrogenation and hydroisomerization reactions at 300 degrees C and 350 degrees C, respectively.