The host-guest interaction between four hexaaza macrocyclic ligands (3,6,9,17,20,23-hexaazatricyclo[23.3.1.1(11,15)]-triaconta-1(29),11,13,1 5 (30),25(27)-hexaene (Bd), 3,6,9,16,19,22-hexaazatricyclo[22.2.2.2(11,14)]triaconta-1(27),11-(30),12,14(29),24(28),25-hexaene (P2), 3,7,11,19,23,27-hexaazatricyclo[27.3.1.1(13,17)]tetratriaconta-1(33),13, 15,17(34),29,31 -hexaene (Bn), 3,7,11,18,22,26-hexaazatricyclo[26.2.2.2(13,16)]tetratriaconta-1 (31),13(34),14,16(33),28(32),29-hexaene (P3)) and two dicarboxylic acids (oxalic acid, H(2)Ox; oxydiacetic acid, H(2)Od) have been investigated using potentiometric equilibrium methods. Ternary complexes are formed in aqueous solution as a result of hydrogen bond formation and Coulombic interactions between the host and the guest. In the [(H(6)P2)(OX)](4+) complex those bonding interactions reach a maximum yielding a log K-6(R) of 6.08. This species has been further characterized by means of X-ray diffraction analysis showing that the oxalate guest molecule is situated inside the macrocyclic cavity of the P2 host. X-ray diffraction analysis has also been carried out for the complex [(H6Bn)(Od)(2)](Br)(2).6H(2)O, where now the oxydiacetate is bonded to the host but outside the macrocyclic cavity. Competitive distribution diagrams and total species distribution diagrams are used to graphically illustrate the most salient features of these systems, which are the following: (a) The Bd and P2 ligands bind Ox significantly much more stronger than Od. This is clearly manifested for the P2:Ox:Od competitive system, where a selectivity of 92.5% in favor of the P2-Ox complexation against P2-Od is obtained at p[H] = 2.8. (b) No isomeric effect is found when comparing binding capacities of oxalate with two isomeric ligands such as P2 and Bd since their affinity to bind the substrate is relatively similar. (c) Bn and P3 ligands have a similar behavior as described in (a) for P2 and Bd except that due to the increase of cavity size the differentiation becomes smaller. (d) Less basic ligands containing two methylenic units Bd (log beta(6)(H) 6 = 40.42) and P2 (40.42) bind stronger to the substrates than those containing three methylenic units Bn (50.32) and P3 (50.64) even though their relative predominance depends on p[H].