The complex formation of Hg2+ ion with some benzo-substituted macrocyclic diamides in binary acetonitrile + water mixtures was studied by differential pulse polarography at 25 degrees C. The stoichiometry and stability of the complexes were determined by monitoring the shift in the Hg2+ differential pulse peak potential against the macrocycle concentration. In all cases studied, it was found that the stability of the resulting 1:1 complex decreases drastically by increasing the amount of water in the binary mixtures. The observed stability order in a given solvent mixture is discussed in terms of the cavity size, structural flexibility and nature of the substituents on the macrocyclic ring.