This paper describes the studies on kinetics of bio-dissolution of valuable metals such as copper, nickel and cobalt from the Indian Ocean nodules using Thiobacillus ferrooxidans and Thiobacillus thiooxidans in the presence of pyrite and sulphur. The bacterial oxidation of pyrite and sulphur produces ferrous sulphate and sulphurous acid respectively which reduce Mn(IV) of the nodules to Mn(II), thereby dissolving the metals through indirect mechanism. The leaching data collected in the temperature range 293-308 K at a pH of 2 and 5% pulp density with the particle size of 300 to -75 mum, show that the dissolution of the metals follows shrinking core kinetic model. The leaching is controlled by the diffusion of the lixiviant through the product layer which is also indicated by the activation energy values. (C) 2003 Elsevier Science Ltd. All rights reserved.