The structures of the anhydrous 1:1 proton-transfer compounds of the dye precursor aniline yellow [4-(phenyldiazenyl)aniline], namely isomeric 4-(phenyldiazenyl)anilinium 2-carboxy-6-nitrobenzoate, C12H12N3+·C8H4NO6, (I), and 4-(phenyldiazenyl)anilinium 2-carboxy-4-nitrobenzoate, C12H12N3+·C8H4NO6, (II), and 4-(phenyldiazenyl)anilinium 3-carboxy-5-nitrobenzoate monohydrate, C12H12N3+·C8H4NO6·H2O, (III), have been determined at 130 K. In (I) the cation has longitudinal rotational disorder. All three compounds have substructures comprising backbones formed through strong head-to-tail carboxyl–carboxylate hydrogen-bond interactions [graph set C(7) in (I) and (II), and C(8) in (III)]. Two-dimensional sheet structures are formed in all three compounds by the incorporation of the 4-(phenyldiazenyl)anilinium cations into the substructures, including, in the cases of (I) and (II), infinite H—N—H to carboxylate O—C—O group interactions [graph set C(6)], and in the case of (III), bridging through the water molecule of solvation. The peripheral alternating aromatic ring residues of both cations and anions give only weakly π-interactive step features which lie between the sheets.