The fate of arsenic in the environment is fundamentally linked to its speciation. Arsenic in aerobic environments is predominantly arsenate, however under reducing conditions arsenite species dominate. In sulfidic environments, thioarsenic species are known to exist and play key roles in arsenic speciation. Although thioarsenic species are know to be important in sulfidic environments, little is know about their formation, structure, chemistry, and their stability in the environment. In particular, information is sparse relating to the redox behavior of arsenate and arsenite in the presence of aqueous sulfide and at variable pH. In order to address these questions a series of solutions containing arsenate or arsenite in the presence of sulfide species and at variable pH were analyzed using extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge spectroscopy (XANES) analysis. With these techniques it was possible to access arsenic speciation and obtain kinetic information about the changes in speciation of arsenic with time. Results indicate that both thioarsenite and thioarsenate species exist and both forms of thioarsenic species need to be considered in sulfidic environments. The results of these experiments will be presented. This is an abstract of a proposed presentation and does not necessarily reflect EPA policy.