Abstract Interactions of tetrameta-, hexameta- and octameta-phosphate ions with proton, alkali metal ions and hexamminecobalt(III) ion were studied by the conductivity and potentiometric method. It was shown that the logarithms of thermodynamic stability constants of 1 : 1 proton associated molecules with these cyclic phosphates increase linearly with the anion charges contrary to those with the linear phosphates. The anomalous order of the stability constants obtained for alkali metal ion-pairs with a cyclic phosphate suggests the possibility of partial dehydration of the alkali metal ions with smaller atomic weights in the ion-pair formation. By the use of ion-selective electrodes, it was shown that in a solution containing an excess of sodium, potassium or rubidium ions compared with cyclic phosphate ions, more than one cation is associated with a cyclic phosphate ion. The apparent association constants of these highly associated species were determined. Interactions of cyclic phosphate ions with hexamminecobalt(III) ion, which was selected as an example of tervalent cations, are discussed in comparison with those for uni- and bi-valent cation complexes.