We have measured the densities at temperatures T = (278.15 to 363.15) K and heat capacities at T = (278.15 to 393.15) K of aqueous solutions of 18-crown-6 and of (18-crown-6 + KCl) at molalities m = (0.02 to 0.3) mol (.) kg(-1) and at the pressure 0.35 MPa. We have calculated apparent molar volumes V-phi and apparent molar heat capacities C-p,C-phi, for 18-crown-6(aq), and we have applied Young's Rule and have accounted for chemical speciation and relaxation effects to resolve V-phi and C-p,C-phi for the (18-crown-6: K+,Cl-)(aq) complex in the mixture. We have also calculated estimates of the change in volume Delta,V-m, the change in heat capacity Delta(r)C(p.m), the change in enthalpy Delta(r)H(m) and the equilibrium quotient log Q for formation of the complex at T = (278.15 to 393.15) K and m = (0 to 0.3) mol (.) kg(-1). (C) 2004 Elsevier Ltd. All rights reserved.