Association constants of the 1:1 complexes of a series of pyrido crown ethers 1 (n=0–6) and a number of large ring (27–33 ring atoms) benzo and dibenzo crown ethers 2,3 with alkali, barium and guanidinium chlorides have been determined potentiometrically in MeOH. For most of the ions besides a global maximum for 18-membered rings, a local maximum for the complexation with 30-membered rings is observed. The appearance of local maxima is attributed to the tennisball-seam like conformations that large macrocycles can adopt. The larger crown ethers show a remarkbly high K+ /Na+ selectivity similar to the antibiotic valinomycin, a structurally related 36-membered macrocycle. Therefore, these large ring crown ethers can be regarded as synthetically easy to modify valinomycin mimics. The effects of the solvation of the hosts, guests and complexes on the overall (co-)complexation is discussed. Comparison of data from extraction experiments with association constants clearly shows that differences in the distribution of the complexes between the aqueous and organic phases may be a dominating factor for the selective extraction of cations.