Potentiodynamic sweeps, cyclic voltammetry, and the rotating ring-disk technique were used to study the anodic dissolution of pyrite. Pyrite behaves as a metal in the potential region investigated. The dissolution was shown to be kinetically controlled. At low overpotentials a total of three electrons were transferred and selective dissolution of Fe3+ occurs with the formation of a metal-deficient sulfide layer. At large overpotentials this passivating sulfide layer is oxidized in an overall 15 electron-transfer reaction. Oxygen reduction at pyrite was studied in the pH range 1 to 3 using the rotating ring-disk technique. Hydrogen peroxide was detected at the ring and mechanistic analyses of the data show that the reaction can proceed by two paths, one involving solution-soluble hydrogen peroxide and one involving only adsorbed intermediates. The amount of hydrogen peroxide formed during the course of reaction diminishes as the pH decreases.