Theoretical equations were derived for the determination of ion-pair formation constants (K-0 in mol(-1).dm(3)) for 2:2 (M2+X2-) and 2:1 electrolytes and NIX with crown ethers (L) in water at an ionic strength of zero. The K-0 values were determined potentiometrically at 25 degrees C; CdSO4, CdCl2, Cd(picrate)(2), CaCl2, and Ca(picrate)(2) were used as the electrolytes, and CdL2+SO42- with L = 18-crown-6 ether (18C6) and benzo-18C6 were used as the ion-pair complexes. By examining the validity of the equations directly or indirectly, the K-0 values for CdSO4 and CdCl2 were compared with the corresponding literature. Consequently, it was evident that the derived equations are useful for the determination of K-0.