The aqueous chemistry of pyride (FeS2) is considered, with emphasis on the relationship between the semiconductor properties of the solid and the characteristics of the interfacial oxidative decomposition reactions. With the aid of energy level correlations which compare the band edges of pyrite with the energy levels of selected redox couples, the relative importance of valence band holes and conduction band electrons in the corresponding interfacial charge transfer reactions is ascertained. Energy level correlations are also presented which show that the oxidative decomposition of pyrite via a hole pathway is thermodynamically feasible in both acidic and basic solutions. Electrochemical mechanisms are presented which yield rate laws that are consistent with published kinetic data. The upper region of the valence band of pyrite consists of completely filled iron d-orbitals and these impart catalytic properties to the solid surface. The critical role played in the oxidative decomposition process by the interaction between the surface Fe d-states and OH2 and OHMIN species in the aqueous phase is highlighted. Implications for environmental control and waste utilization are discussed.