Crystallographic characteristics of congruent melting 18-crown-6 crystalline hydrates were specified using X-ray structure analysis. Melting enthalpies of 18-crown-6 tetra- and hexahydrates have been determined; melting points of these compounds have been specified. Liquidus of 18-crown-6-water system has been calculated in the concentration range x(H2O) = 0.750-0.935 by using the received data and known thermodynamic properties of the solution. (C) 2010 Elsevier B.V. All rights reserved.