The complexation processes between Li+, Na+, K + and NH4+ cations with macrocyclic ligand, 4′-nitrobenzo-15C5, were studied in acetonitrile-methanol (AN-MeOH) binary mixtures at different temperatures using conductometric method. The conductance data show that the stoichiometry of the complexes formed between the ligand and Li+, Na+, K+ and NH4+ cations is 1:1(M:L). Addition of 4′-nitrobenzo-15C5 to these cations solution, causes a continuous increase in the molar conductivities which indicates that the mobility of the complexed cations is more than the uncomplexed ones. The values of stability constants of the complexes were determined from conductometric data using GENPLOT computer program. The obtained results show that the selectivity order of the ligand for Li+, Na+, K+ and NH4+ cations changes with the nature and composition of the binary mixed solvent. The values of thermodynamic parameters (ΔH°c, ΔS°c) for formation of the complexes were obtained from temperature dependence of the stability constants using the van\\\\\\\\\\\\\\\'t Hoff plot. The results show that the complexes are both enthalpy and entropy stabilized. A non-linear behavior was observed between the stability constants (log Kf) of the complexes and the composition of the AN-MeOH binary solution.

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