A potentiometric study on the interaction at 25 °C and 0.15 mol dm −3 between large polyazacycloalkanes of the series [3 k]aneN k and Zn(II) and Cd(II) ions in dmso:H 2O (80:20 vol./vol.) is reported. The protonation behavior of the free macrocycles and coordination trends are compared with those previously observed in aqueous solution. The main features observed in the mixed solvent are: similar protonation behavior of the free polyamines, greater tendency to form binuclear and protonated complexes and a lower tendency to bear hydrolytic reactions.