Soluble arsenic-sulfur compounds play important roles in the biogeochemistry of arsenic in sulfidic waters but conflicting analytical evidence identifies them as either thioarsenates (AsV-sulfur species) or thioarsenites (AsIII-sulfur species). Here, we present the first characterization of thioarsenates (mono-, di-, and tetrathioarsenate) by X-ray absorption spectroscopy and demonstrate that their spectra are distinctly different from those of AsIII-sulfur species, as well as from arsenite and arsenate. The absorption near edge energy decreases in the order arsenate > thioarsenates > arsenite > AsIII-sulfur species, and individual thioarsenates differ by 1 eV per sulfur atom. Fitted AsV-S and AsV-O bond distances in thioarsenates (2.13-2.18 Å and 1.70 Å, respectively) are significantly shorter than the corresponding AsIII-S and AsIII-O bond distances in AsIII-S species (2.24-2.34 Å and 1.78 Å, respectively). Finally, we demonstrate that thioarsenates can be identified by principal component analysis in mixtures containing AsIII-sulfur species. This capability is used to study the spontaneous reduction of tetrathioarsenate to AsIII-sulfur species (possibly trithioarsenite) upon acidification from pH 9.5 to 2.8.