The role of interfacial iron and sulphur species in pyrite (FeS2) photoelectrochemistry was examined. It was found that water plays a crucial role in liberating oxidizable iron and in leading to the formation of reducible sulphur at the pyriteelectrolyte interface. The amino acid cysteine interacts both with the interfacial iron and the sulphur centres forming complexes. It enhances oxidation via the iron and decreases reduction via the sulphur species. The energetic and molecular mechanisms are discussed. A significant increase of photoeffects due to cysteine interaction indicates that surface recombination is decreased by facilitating electron transfer via -S- and -S-S- bridges which interact electronically with the Fe-eg and Fe-t2g energy band states of pyrite. Cysteine modified pyrite interfaces may therefore serve as simplified systems for modelling biological FeS, Fe2S2 and Fe4S4 iron sulphur centres.