Vapor pressures of binary aqueous solutions containing 18-crown-6 over a wide concentration range up to saturation were measured at (293.15 and 298.15) K, and activities of solvent were calculated. Melting enthalpies of 18-crown-6 octa- and dodecahydrates have been determined by the differential scanning calorimetry (DSC) method. The new experimental data and previously published literature data (activities of water, enthalpies of dilution, and solubility data) were used for thermodynamic modeling of the water-18-crown-6 system. As a result the phase diagram of the water-18-crown-6 system was constructed using the universal quasichemical (UNIQUAC) expression for the excess Gibbs energy of the liquid phase.