The geometric and electronic structures of FeS2 (100) surface have been studied by a quantum-mechanical calculation using a total-energy pseudopotential code, CASTER The (100) surface is very stable and does not give any significant geometric relaxation. The electronic structure of FeS2 (100) surface is characterized by the appearance of new native surface states in the bulk band gap, which correspond to antibonding mixed Fea-Ssp(3) states. These surface states play an important role as mediators of electron transfer on both anodic and cathodic sites in the incipient oxidation of pyrite. Moreover, the (100) surface has small band gaps and shows some metallic character. It is predicted that the rate of cathodic reductive reaction of O-2 in the incipient oxidation of pyrite is much faster than previously considered. The transport of electrons from the anodic sites to the cathodic sites on the (100) surface is faster and hole injection of anodic sites is not the rate-determining step. So we can deduce that the rate-determining step of incipient oxidation for pyrite consists of both electron transfer of pyrite/aqueous O-2 interface and the splitting of H2O. (C) 2003 Elsevier Inc. All rights reserved.