The extraction of molybdenum and cobalt from spent petroleum catalyst (Co-Mo/Al2O3) was investigated using sulphuric and nitric acid solution. Direct leaching of spent catalyst with sulphuric acid was not effective, whereas the combination of sulphuric and nitric acid was significant for the recovery of both molybdenum and cobalt. The effect of experimental factors such as reaction time, acid concentration, temperature, solid-liquid ratio and particle size were studied to determine the best conditions for the solubilisation of metal values. The parameters, temperature and nitric acid concentration are found to be critical factors especially on leaching of molybdenum from the spent catalyst. Under optimum conditions (pulp density 10% (w/v), [H2SO4] 0.5 M, [HNO3] 4.0 M. particle size 51-70 mu m, temperature 50 degrees C and time 5 h), about 99.7% of molybdenum and 99.6% of cobalt could be extracted with 14.9% extraction of aluminium. (c) 2012 Elsevier B.V. All rights reserved.