The interaction of mono-, di-, and tripolyphosphate anions with the various protonated forms of the hexaaza macrocyclic ligand BMXD, 3,6,9,17,20,23-hexaazatricyclo[23.3.1.1(11.15)]triaconta-1(29),11(30),12,14,25,27-hexaene, was investigated by potentiometric equilibrium studies. The protonated ligand forms a number of stable binary complexes in solution with each of the anionic substrates and the strength of interaction increases in the order mono- < di- < tripolyphosphate. Formation constants for all the species found are reported and compared with those of analogous systems. The hexahydrobromide salt of BMXD, (H(6)BMXD)Br-6 . 7H(2)O crystallizes in the orthorhombic system, space group Pna2(1), with a = 11.597(4) Angstrom, b = 12.604(4) Angstrom, c = 27.361(7) Angstrom, and Z = 4. The binary complex (H(4)BMXD-H2P2O7)Br-2 . 5.2H(2)O crystallizes in the monoclinic system, space group C2/c, with a = 28.194(6) Angstrom, b = 13.875(3) Angstrom, c = 20.811(4) Angstrom, beta = 113.72(3)degrees, and Z = 8. The X-ray structure results reveal the nature of the interaction between the protonated macrocycle and anionic substrates to be that of multiple hydrogen bonding.