Car-Parrinello simulations have been performed to study the interaction of water with pyrite (100) surface. The stability and the structural and electronic properties of both the molecular and dissociative adsorptions have been addressed. We found a very strong preference for molecular adsorption on the surface iron sites, in agreement with experiment. The dissociative chemisorption of water is energetically disfavored and is even locally unstable; the dissociated fragments transform back to the stable molecular form in a short molecular dynamics run. The calculations revealed that hydrogen bonding plays an important role in the stabilization of the adsorbed water for both the molecular and the dissociative states. We have shown that water forms a coordinative covalent bond with the surface iron atoms by donating electron to the empty iron d(z)(2) orbitals which are the lowest empty states on the clean surface. At full coverage, the sulfur 3p states thus become the lowest available empty states and therefore the subject of possible electron-transfer reactions. (C) 2003 American Institute of Physics.