p-Arsanilic acid forms an isotypic set of three compounds with the alkali metals K, Rb and Cs, in which the primary layered coordination polymeric structures have similar asymmetric units comprising two independent and different metal cations and a bridging water molecule which lie within crystallographic mirror planes parallel to (100). The layers are linked across [100] through amine N—H⋯O hydrogen bonds to arsonate and water O-atom acceptors, giving overall three-dimensional network structures.