The leaching kinetics of spent nickel oxide catalysts in HCl and aqua regia solution is described. The considered parameters affecting nickel recovery are particle size, reaction time, acid concentration, and reaction temperature. Nickel could be extracted with HCl by similar to 87.41?% and with aqua regia by similar to 81.01?% using a spent catalyst with 200 mesh particle size at a reaction temperature of 100 degrees C. The reaction time was 360?min and the concentrations of HCl and aqua regia were 37?% and 80?%, respectively. The leaching kinetics indicates that shrinking sphere and diffusion were the rate-controlling processes during the reactions with HCl and aqua regia, respectively. The activation energies for HCl and aqua regia were determined as 24.04 and 19.03?kJ?mol1, respectively.