Pyrite dissolution in acidic solution was found to occur via both electrochemical oxidation and chemical decomposition. The mechanism of chemical decomposition of pyrite in acidic solution may involve surface complexation of hydrogen ions. The anodic current of pyrite was observed to be of negligible magnitude in acetonitrile solution compared with that in aqueous solution, which indicated that direct reaction of the holes with S-2(2-) in the pyrite lattice was not significant and that the dissolution of pyrite required the presence of water. The anodic dissolution products of pyrite in acidic aqueous solution included elemental sulfur which was detected by x-ray diffraction.