Abstract <p>Data describing effect of composition of aqueous ethanol solvent on the solvation state, acid-base properties, and complexation of 1-ethyl-2<i>H</i>-imidazoline-2-thione (1EI) have been presented. The systematization of experimental results obtained by interphase distribution have shown that the solvation of 1EI in water–ethanol binary solvent decreases at low content of non-aqueous solvent and next smoothly increased. The dependence of p<i>K</i>_α = <i>f</i>(χ_EtOH, mol. fr.) for 1EI is extremal and has a minimum in concentration region of 0.1 mol. fr. of ethanol. On the whole, the basicity of 1EI changes insignificantly on passing from water to aqueous ethanol solvent. Formation constants for silver(I) complexes in aqueous ethanol solvents of variable composition have been determined by potentiometry using silver electrode. Silver(I) complexes with 1EI in aqueous ethanol systems show distinct extremal dependence of <InlineEquation ID="IEq1"> <EquationSource Format="TEX">\(\log \beta _{i}^{0}\)</EquationSource> <!--InrgChem2560387Safarmamadzoda-m1--> </InlineEquation> vs. mole fraction of ethanol. The analysis of solvation contribution of initial compounds and reaction product into formation of monosubstituted complex have shown that the stability of complex [Ag(1EI)]⁺ is considerably determined by the change in solvation environment of complex species, the contribution of difference in thermodynamic functions of ion transfer of {Δ_tr<i>G</i>_Ag+ – Δ_tr<i>G</i>_1EI} into total effect is small.</p>