A conductance study of the interaction of NH4+ and Tl+ ion with 18-crown-6 (18C6), ditertbutyl-dicyclohexyl-18-crown-6((t-but)2DC18C6), diaza-15-crown-5 (DA15C5), dibenzo-21-crown-7 (DB21C7) and dibenzo-24-crown-8 (DB24C8) in acetonitrile meth anol mixture was carried out at 25.0 ± 0.1°C. The formation constants of the resulting 1:1 complexes were determined from the molar conductance-mole ratio data and found to vary in the order of DA15C5 > 18C6 > DB21C7 > (t-but)2DC18C6 > DB24C8, in the case of Tl+ ion in 100% MeOH, and in the or der of: DA15C5 > 18C6 > (t-but)2DC18C6 DB21C7 > DB24C8 for the NH4+ ion in 100% AN and AN-MeOH mixtures. The stabili - ties of the complexes for NH4+ and Tl+ ions varied inversely with the Gutmann donicity of the solvent with the exception of DA15C5-Tl+ complexes. The influence of substitu - ents on the polyether ring and cavity size is discussed
