The kinetics of spent nickel oxide catalyst (NiO/Al2O3) leaching in sulphuric acid solutions was investigated. The effects of sulphuric acid concentration, temperature, stirring speed, and particle size on the rate of nickel leaching were studied. In addition, the reaction residues at various levels of nickel extraction were examined by SEM, X-ray diffraction, electron microprobe, and chemical analysis. The results of the kinetic analysis of the leaching data for various experimental conditions indicated that the reaction is controlled by diffusion through the catalyst network with the activation energy of 16.6 +/- 0.9 kJ/ mol. A linear relationship between the rate constant and the inverse square of the initial particle diameter is also characteristic for a diffusion-controlled process. (c) 2005 Elsevier B.V. All rights reserved.