Electrochemical characterizations of high purity natural crystals of orpiment (As2S3) and realgar (As2S2) were performed in the presence of a bacteria-free, acidic bacterial growth medium for Thiobacillus ferrooxidans, name M2. A carbon-paste electrode (CPE), composed of a mixture of pure mineral grains with graphite powder and silicon oil as a binder was used for the experiments. Reproducibility of the mineral surfaces was assured by monitoring the open-circuit potential (OCP) of the initial CPE-mineral-electrolyte interface. The voltammetric study revealed that the oxidation processes for both of these arsenic-bearing minerals involves two steps : an initial oxidation of the sulfide mineral to H3AsO3 and HSO4, followed by the oxidation of H3AsO3 to H3AsO4. The reduction processes for the two minerals, however, differ. While the reduction of orpiment occurs in a dual step process, for realgar, the process is achieved in a single step.