The complex formation constants of sodium ions with 18-crown-6 in mixtures of water and butan-1-ol or butan-2-ol at 298.15 K were determined by the potentiometric method with the use of sodium and chloride ion-selective electrodes. The correlations between the logarithm of the complex formation constants and the mole fraction of an alcohol in the mixed solvent were constructed based on the published data and the experimental results obtained in the present study. It was shown that the numerical value of the constant is to a great extent determined by the water-to-alcohol ratio and is virtually independent of the nature of the alcohol. The proposed correlations are suitable for the estimation of the stability constants of 18-crown-6 with sodium, potassium, and thallium ions in mixed aqua-alcohol solvents.