Pyrite (FeS2) is a common and ubiquitous constituent of reservoir rocks and forms geologically under extremely reducing conditions. Pyrite is easily oxidized by exposure to moderately oxidizing conditions, including de-oxygenated injection or :process water. When pyrite is oxidized in water, the resulting products include sulphuric acid (H2SO2) and sulphate ion (SO(4)2-). In the near well bore, the resulting waters are far from equilibrium with Portland type well cements. A small amount of pyrite oxidation can result in total removal of well cement and the replacement of the cement by structurally less stable products, such as calcium sulphate. In addition, the sulphuric acid may result in corrosion of well tubing or above ground facilities. Casing leaks, particularly in injection wells? may lead to failures of casing due to cement collapse, tubing corrosion, or both. In the reservoir, during water flooding or steam injection, pyrite: oxidation may release sulphate ion (SO42-) which can promote the activity of sulphate reducing bacteria, resulting in "souring" and the production of the acids gases H2S and CO2, where no production of acid gas was noted before injection. The results of pyrite oxidation are examined using reaction path modeling, in which the effects on reservoir fluids and rocks of these complex chemical reactions, including corrosion and scaling, can be determined.