A new iron(III) arsenate templated by ethylenediamine, (C2H10N2)[Fe(HAsO4)(2)(H2AsO4)](H2O), has been prepared by hydrothermal synthesis. The unit-fell parameters are a 8.705(3) Angstrom, b = 16.106(4) Angstrom, c = 4.763(1) Angstrom, beta = 90.63(3)degrees; monoclinic, P2(1) with Z = 2. The compound exhibits a chain structure along the c-axis with the ethylenediammonium cations as counterion. The chains show isolated FeO6 octahedra with two HAsO4 and one H2AsO4 tetrahedra per FeO6 octahedron. The ESR spectrum at 5.0 K is isotropic with a g-value of 2.0, which remains practically unchanged at room temperature. Magnetic measurements indicate the presence of antiferromagnetic interactions. A value of -0.835 K for the J-exchange parameter has been calculated by fitting the magnetic data to a model for antiferromagnetic chains of spin S = 5/2.