Ion-pair formation constants (K-MLX in mol(-1) (.) dm(3)) for M(B18C6)(+), M(15C5)(+), or M(B15C5)(+) with pairing anions (X-) in water were determined by potentiometry with ion-selective electrodes at 25 degrees C over wide ranges of the ionic strength (I). The symbols B18C6, 15C5, and B15C5 denote benzo-18-crown-6 ether, 15-crown-5 ether, and benzo-15C5, respectively; metal salts, MX, used are sodium picrate (NaPic), KPic, NaBPh4, NaMnO4, and KMnO4. The K-MLX values at I = 0 mol dm(-3) (K(MLX)degrees) were evaluated from analyzing the I dependence of the log K-MLX values determined. Then, effects of shapes of X-, the benzo group of the ethers, their ring sizes, and the shielding of the metal ions, M+, by the ethers on these K(MLX)degrees values were discussed in comparison with the values previously reported on M(18C6)X, M(15C5)Pic, and M(Bl5C5)Pic. Also, center-to-center distances in the M(18C6 derivatives)(+)-X- or M(15C5 ones)(+)-X- pairs were estimated from Bjerrum's equation.