4-Amino-trans-azobenzene {or 4-[(E)-phenyldiazenyl]aniline} can form isomeric salts depending on the site of protonation. Both orange bis{4-[(E)-phenyldiazenyl]anilinium} hydrogen phosphate, 2C12H12N3+.HPO4(2-), and purple 4-[(E)-phenyldiazenyl]anilinium dihydrogen phosphate phosphoric acid solvate, C12H12N3+.H2PO4-.H3PO4, (II), have layered structures formed through O-H...O and N-H...O hydrogen bonds. Additionally, azobenzene fragments in (I) are assembled through C-H...pi interactions and in (II) through pi-pi interactions. Arguments for the colour difference are tentatively proposed.