In this study we investigate a new family of arsenate-bearing phases belonging to one set of structures (Basic Ferric Sulfate-BFS: monoclinic–orthorhombic FeOHSO₄) that are significant as an industrial arsenic control material in the environment or a cathodic material in rechargeable Li-ion battery cells. We determine for the first time (after two decades of its known existence in the processing industry) the average crystallographic structure of Basic Ferric Arsenate Sulfate-BFAS: Fe(AsO₄)_1−x(SO₄)_x(OH)_x·(1−x)H₂O) that is a member of this family of phases and how it relates to its parent BFS structure. Moreover, we demonstrate how the substitution of AsO₄ ↔ SO₄ affects the crystallographic structure of these phases, the phase(s) that are formed and their material properties as environmentally stable arsenic controls or Li-ion battery cathodes.