Formation of Fe(II)-As(III) solids is suspected to limit dissolved As concentrations in anaerobic environments. Iron(II) precipitates enriched in As(III) have been observed after microbial reduction of As(V)-loaded lepidocrocite (γ-FeOOH) and symplesite (Fe(II)^sub 3^(As(V)O^sub 4^)^sub 2^·8H^sub 2^O) and upon abiotic reaction of Fe(II) with As(III). However, the conditions favorable for Fe(II)-As(III) precipitation and the long-term stability (relative to dissolution) of this phase are unknown. Here we examine the composition, local structure, and solubility of an Fe(II)-As(III) precipitate to determine environments where such a solid may form and persist. We reveal that the Fe(II)-As(III) precipitate has a composition of H^sub 7^Fe^sub 4^(AsO^sub 3^)^sub 5^ and a log K^sub so^ of 34 for the dissolution reaction defined as: H^sub 7^Fe^sub 4^(AsO^sub 3^)^sub 5^ + 8H^sup +^ = 4Fe^sup 2+^ + 5H^sub 3^AsO^sub 3^. Extended X-ray absorption fine structure spectroscopic analysis of H^sub 7^Fe^sub 4^(AsO^sub 3^)^sub 5^ shows that the molecular environment of Fe is dominated by edge-sharing octahedra within an Fe(OH)^sub 2^ sheet and that As is dominated by corner-sharing As^sup III^O^sub 3^ pyramids, which are consistent with previously published structures of As(III)-rich Fe(II) solids. The H^sub 7^Fe^sub 4^(AsO^sub 3^)^sub 5^ solid has a pH-dependent solubility and requires millimolar concentrations of dissolved Fe(II) and As(III) to precipitate at pH <7.5. By contrast, alkaline conditions are more conducive to formation of H^sub 7^Fe^sub 4^(AsO^sub 3^)^sub 5^; however, a high concentration of Fe(II) is required, which is unusual under alkaline conditions.