The compounds Eu₃Cl₂[AsO₃][As₂O₅] and Gd₃Cl₂[AsO₃][As₂O₅] are part of a series of isotypic lanthanoid(III) chloride oxoarsenates, crystallizing in the triclinic space group P̅1 and sharing structural similarities with Sm₃Cl₂[AsO₃][As₂O₅]. Synthesized via solid-state reactions in silica ampoules at approximately 800 °C, these compounds remain an important reference for understanding the structural and chemical behavior of this material class.