Leaching of nickel from a hydrodesulphurization spent catalyst generated from the sulfur removal process of petroleum refineries was investigated using ammonium sulfate ((NH4)(2)SO4) solution. The spent catalyst used in the study was composed of vanadium and nickel as sulfides, supported in an alumina matrix. Experiments were carried out in a stirred batch reactor under the temperature range of 298-368 K and the (NH4)(2)SO4 solution of 1.3-3.3 mol/dm(3). As a result, about 94.0% of nickel was leached from the 2.6 mol/dm(3) (NH4)(2)SO4 solution in 90 min at the temperature of 368 K. A shrinking core model was found to be useful in describing the rate of nickel leaching. The rate of nickel leaching is limited by ash layer diffusion. The nickel leaching is of the first order with respect to the (NH4)(2)SO4 concentration and has an activation energy of 16.2 kJ/mol.