The complex formation between La3+, UO 22+ Ag+, and NH 4+ cations and macrocyclic ligand, dicyclohexyl-18-crown-6 (DCH18C6), was studied in acetonitrile-tetrahydrofuran (AN-THF) binary mixtures at different temperatures using the conductometric method. The results show that with the exception of complexation of the NH 4+ cation with DCH18C6 in pure acetonitrile, the stoichiometry of all the complexes is being 1: 1 (M: L). The stability constants of the complexes were determined using a GENPLOT computer program. The nonlinear behavior which was observed for changes of log K f of the complexes versus the composition of the mixed solvent was discussed in terms of solvent-solvent interaction in their binary solution, which results in changing the chemical and physical properties of the constituent solvents when they mix with one another and, therefore, changing the solvation capacities of the metal cations, crown ether molecules, and even the resulting complexes with changing the mixed solvent composition. The results show that the selectivity of DCH18C6 for the studied cations changes with the composition of the AN-THF binary system. The sequence of stabilities of complexes in an AN-THF binary solution (mol. % AN = 75.0) at 25°C is [(DCH18C6)La)]3+ > [(DCH18C6)UO2]2+ > [(DCH18C6)Ag]+ ̃ [(DCH18C6)NH4]+, but in the case of other binary systems of AN/THF (mol. % AN = 25.0 and 50.0) is [(DCH18C6)La]+ > [(DCH18C6)NH4]+ ̃ [DCH18C6)UO2]2+ > [(DCH18C6)Ag]+.

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