Structures having the unusual protonated 4-arsonoanilinium species, namely in the hydro­chloride salt, C6H9AsNO3+·Cl−, (I), and the complex salts formed from the reaction of (4-amino­phenyl)­arsonic acid (p-arsanilic acid) with copper(II) sulfate, i.e. hexa­aqua­copper(II) bis­(4-arsonoanilinium) di­sulfate dihydrate, (C6H9AsNO3)2Cu(H2O)62·2H2O, (II), with copper(II) chloride, i.e. poly[bis­(4-arsonoanilinium) [tetra-μ-chlorido-cuprate(II)]], {(C6H9AsNO3)2[CuCl4]}n, (III), and with cadmium chloride, i.e. poly[bis­(4-arsonoanilinium) [tetra-μ-chlorido-cadmate(II)]], {(C6H9AsNO3)2[CdCl4]}n, (IV), have been determined. In (II), the two 4-arsonoanilinium cations are accompanied by [Cu(H2O)6]2+ cations with sulfate anions. In the isotypic complex salts (III) and (IV), they act as counter-cations to the {[CuCl4]2−}n or {[CdCl4]2−}n anionic polymer sheets, respectively. In (II), the [Cu(H2O)6]2+ ion sits on a crystallographic centre of symmetry and displays a slightly distorted octahedral coordination geometry. The asymmetric unit for (II) contains, in addition to half the [Cu(H2O)6]2+ ion, one 4-arsonoanilinium cation, a sulfate dianion and a solvent water molecule. Extensive O—H...O and N—H...O hydrogen bonds link all the species, giving an overall three-dimensional structure. In (III), four of the chloride ligands are related by inversion [Cu—Cl = 2.2826 (8) and 2.2990 (9) Å], with the other two sites of the tetra­gonally distorted octa­hedral CuCl6 unit occupied by symmetry-generated Cl-atom donors [Cu—Cl = 2.9833 (9) Å], forming a two-dimensional coordination polymer network substructure lying parallel to (001). In the crystal, the polymer layers are linked across [001] by a number of bridging hydrogen bonds involving N—H...Cl inter­actions from head-to-head-linked As—O—H...O 4-arsonoanilinium cations. A three-dimensional network structure is formed. CdII compound (IV) is isotypic with CuII complex (III), but with the central CdCl6 complex repeat unit having a more regular M—Cl bond-length range [2.5232 (12)–2.6931 (10) Å] compared to that in (III). This series of compounds represents the first reported crystal structures having the protonated 4-arsono­anilinium species.