Bioleaching studies were conducted to evaluate the recovery of metal values from waste petroleum catalyst using two different acidophilic microorganisms, Acidithiobacillus ferrooxidans and Acidithiobacillus thiooxidans. Various leaching parameters Such as contact time, pH, oxidant concentration. pulp densities, particle size, and temperature were studied in detail. Activation energy was evaluated from Arrhenius equation and Values for Ni, V and Mo were Calculated in case of both the acidophiles. In both cases, the dissolution kinetics of Mo was lower than those of V and Ni. The lower dissolution kinetics may have been due to the formation of a sulfur product layer, refractoriness of MoS2 or both. Multivariate statistical data were presented to interpret the leaching data in the present case. The significance of the leaching parameters was derived through principle component analysis and Multi linear regression analyses for both iron and sulfur oxidizing bacteria. (C) 2009 Elsevier B.V. All rights reserved.