Two novel 3,12- and 3,7-bis(4′-nitrophenyl)-azo-5,5,10,10,15,15,20,20-octamethyl calix[4]pyrroles were prepared and studied as potential anion binders for AcO⁻ and H₂PO₄⁻; the isomeric pair not only allowed for the colorimetric detection but also helped to discriminate the geometrically different anions.