Ion-pair formation constants (K(n)(0), n = 1, 2) of Na(2)SO(4), Na(2)S(2)O(3), Na(2)CrO(4), CaCl(2), CdCl(2), CdBr(2), and Cdl(2) in water were determined at 298 K and an ionic strength of 0 mol.dm(-3) by potentiometry with Na(+)-, Cl(-)-, Br(-)-, and I(-)-selective electrodes. Simultaneously, equations for the K(n)(0) determination based on the DeFord-Hume method were derived. The thus-obtained K1 values for Na2SO4, Na2S203, CdCl(2),, and CaCl(2), were compared with those reported before, and the validity of their values was confirmed. interactions of Na(+) with oxo anions (X(2-)) and Cd(2+) with the halide ions in water were characterized by considering hydration to and Coulombic forces acting between these ions. Then, a difference in K(1)(0) between CdI(+) and NaSO(4)(-) was interpreted as that in the Coulombic force between the pairing ions. Also, the K(1)(0) difference between Na(+)BPh(4)(-) and Ph(4)As(+)Cl(-) or CaCl(+) and NaSO(4)(-) was examined by using a thermodynamic cycle of these salts in a water/nitrobenzene (NB) distribution system. Furthermore, the distribution constants in the NB phases were evaluated.