The current trend in the processing of refractory gold and base metal ores is to subject the concentrates to an oxidative pressure acid leach (PAL), 1-8 prior to metal ion collection. For gold collection, PAL might be followed by cyanidation. The purpose of the PAL step is most often to destroy and oxidize encapsulating or associated mineral sulfide matrices which would otherwise reduce metal ion recoveries in subsequent collection steps. A common sulfide mineral present in gold and base metal concentrates is pyrite (FeS 2 ). Often the water used to make the pulps that are treated by PAL comes from groundwater sources close to mine sites. This water frequently contains high levels of chloride salts. Consequently, the PAL process, and thus the oxidation of minerals such as pyrite, may be carried out in the presence of high concentrations of chloride ions. This study focuses on utilizing electrochemical techniques to evaluate the effect of chloride ions on the oxidation of pyrite under acidic conditions.The aqueous electrochemistry of pyrite in acid has been studied in various electrolytes, including H 2 SO 4 , [9][10][11]10,[12][13][14]10,15,16 and HNO 3 . 10,17 Bailey and Peters, 9 who studied the anodic behavior of pyrite in 1.0 M HClO 4 at 110ЊC, proposed that the oxidation of pyrite proceeds via a combination of two pathways which produce both sulfur and sulfate ions, with the latter dominating as the product at higher potentials. They also confirmed from leaching studies, using oxygen-18 as the oxidant, that the oxygen incorporated into the sulfate product is from water and not oxygen, thus proving that pyrite oxidation proceeds via an electrochemical pathway. Biegler and Swift, 10 who investigated the mechanism of pyrite oxidation mainly in sulfuric acid solutions at 25ЊC, concluded that the sulfur and sulfate ions form from two independent pathways described by Reactions 1 and 2. The sulfate route dominated over the potential range accessible at ambient temperature, and the sulfate yield increased linearly with potentialFlatt and Woods 17 investigated the temperature dependence of the yields of sulfur formed after anodic pyrite oxidation in mixed solutions of nitric and sulfuric acid. They also found that the sulfur yields decreased with increasing potential. However, they did not find any discernible change in the potential dependence of the relative yields of sulfur and sulfate in the range of 26 to 80ЊC.Yin et al. 16 have employed a platinum-pyrite ring-disk electrode to study the surface oxidation process of pyrite in hydrochloric solutions. They concluded that in the potential region of 0.4 to 0.6 V vs. saturated calomel electrode (SCE) approximately 50% of the pyrite was oxidized to ferric and sulfate ions, and the other 50% to sulfur and ferrous ions. The latter could be detected by holding the ring at a potential of 1.0 V. As the pyrite disk potential was increased to 1.2 V, more than 90% of the pyrite was oxidized to ferric and sulfate ions, and less than 10% formed ferrous ions. Yin et al. observe...