The title compounds contain the catena-arsenite [AsO(2)](n)(n-) unit, in which the As(III) atom is pyramidally coordinated to one terminal and two bridging O atoms, resulting in an infinite anionic chain. Ethylenediammonium catena-arsenite, (C(2)H(10)N(2))(0.5)[AsO(2)], is the first example of this anion in the company of an organic cation. The ethylenediammonium species interact with the [AsO(...