Abstract <p>The protonation constants of 1-aza-18-crown-6 (1-aza18C6) in water–ethanol and water–dimethyl sulfoxide mixed solvents at 298.15 K were determined by potentiometric titration. An increase in the content of the nonaqueous component in aqueous–organic solutions was found to result in a significant decrease in the basicity of 1-aza-18-crown-6. A similar trend is observed for the protonation constants of some amines in aqueous ethanol solutions. The contributions of reagent solvation to the change in the Gibbs energy of protonation transfer in mixed solvents were analyzed. The solvation contribution of the macrocycle in the free and protonated states, {Δ_tr<i>G</i>(HL) – Δ_tr<i>G</i>(L)}, varies in parallel with the Gibbs energy of proton solvation. In water–dimethyl sulfoxide mixtures, enhanced proton solvation makes a significant contribution to the decrease in the protonation constant. The geometric parameters of 1-aza-18-crown-6 and its protonated form in the free and solvated states were obtained by quantum chemical calculations within the framework of density functional theory (B3LYP/cc-pVTZ). The electron density distribution in the studied species was analyzed within the framework of the NBO scheme. Quantum chemical calculations showed that 1-aza18C6 possesses three stable conformers differing in the position of the hydrogen atom bonded to the nitrogen atom. Protonation of 1-aza18C6 leads to a slight increase in the N–C bond lengths and a shortening of the O–C bonds closest to the nitrogen atom, whereas the remaining internuclear distances change only slightly. The protonation values of <InlineEquation ID="IEq1"> <EquationSource Format="TEX">\({\Delta _{\rm{r}}}H_{298}^0\)</EquationSource> </InlineEquation> were –960.0 and –1185.7 kJ/mol, while the corresponding <InlineEquation ID="IEq2"> <EquationSource Format="TEX">\({\Delta _{\rm{r}}}G_{298}^0\)</EquationSource> </InlineEquation> values were –928.5 and –1154.1 kJ/mol for the free state and the state surrounded by water molecules, respectively. The negative Gibbs energy values indicate that protonation of 1-aza18C6 can occur spontaneously under the conditions considered, which is consistent with the potentiometric titration results.</p>