The title compound α-ammonium iron(III) bis-[hydro-gen arsenate(V)], α-NH4Fe(HAsO4)2 (or poly[ammonium bis-(μ-hydro-gen arsenato)ferrate(III)], {NH4[Fe(HAsO4)2]} n ), syn-the-sized hydro-thermally, is isostructural with NH4Fe(HPO4)2. Condensation of the hydro-gen arsenate groups with FeO6 coordination octa-hedra via common corners results in an overall three-dimensional framework containing inter-connected channels parallel to the a-, b- and c-axis directions. The NH4 (+) cations are located in three inter-secting tunnels, which is promising as an ion exchange. Hydrogen bonding of the types O-H⋯O and N-H⋯O consolidates the packing of the structure. The distortion of the coordination polyhedra is analyzed by means of the effective coordination number and distortion indices. Structural relationships with other compounds of general formula M (I) M (III)[HXO4)]2 (X = P, As) are discussed.