The influence of chloride ions on the electrochemical oxidation of pyrite (FeS2) in sulfuric acid solutions has been studied using ring (glassy carbon)-disk (pyrite) voltammetry and in situ Raman spectroelectrochemistry at ambient temperatures (25 degrees C). The results indicate that at disk (pyrite) potentials greater than 0.7 V vs. saturated calomel electrode, where ring collection is evident, chloride ions inhibit the deposition of elemental sulfur on the pyrite electrode surface by promoting the oxidation of an adsorbed intermediate, believed to be the thiosulfate ion, to soluble tetrathionate ions. In the absence of chloride ions, the thiosulfate intermediate undergoes acid decomposition on the pyrite surface to yield elemental sulfur. During pyrite oxidation, chloride ions are proposed to act as aggressive anions, removing passive layers containing ferric ions by adsorption on the electrode surface. This mechanism is consistent with the increase in die collection of ferric ions at the ring electrode in the presence of chloride ions. The adsorption phenomenon is also proposed to account for the inhibition of pyrite oxidation in the presence of chloride ions.