The kinetics of nickel leaching from spent NiO/Al(2)O(3) catalyst was investigated using sulfuric acid as leaching reagent Leaching experiments were carried out for two different particle sizes of coarse (-8 + 18 mesh) and fine (-400 mesh). Based on the leaching results, a two stage leaching process was observed for the fine particles, one at low Ni recovery (less than 40%) and the other at high recoveries (greater than 80%) while the leaching of coarse particles takes place in a single stage. Kinetic study was performed by fitting the three rate equations proposed in the shrinking core model (SCM) to the leaching data. A new approach was suggested to determine the rate controlling mechanism by applying a constrained multi-linear regression analysis using the least square technique. This technique takes into account all three possible rate controlling mechanisms for leaching processes (i.e., chemical reaction, diffusion through product layer and liquid film mass transfer) simultaneously and determines the controlling step(s) on its own. Results of kinetic study by this method showed that the rate of the first stage of fine particle leaching is controlled by diffusion through the liquid film and the second stage is controlled by chemical reaction on the unreacted core surface. In the case of coarse particles, only chemical reaction control mechanism was observed. (C) 2011 Elsevier B.V. All rights reserved.