We have measured the densities at temperatures T = (278.15 to 368.15) K and heat capacities at T = (278.15 to 393.15) K of aqueous (18-crown-6 + RbCl) and (18-crown-6 + CsCl) at m = (0.02 to 0.33) mol.kg(-1) and at the pressure 0.35 MPa. 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 (18-crown-6-Rb+,Cl-)(aq) and (18-crown-6-Cs+,Cl-)(aq) in the mixtures. We have also calculated values of the change in volume Delta V-r(m), the change in heat capacity Delta C-r(p,m), the change in enthalpy Delta H-r(m), the equilibrium quotient lg Q, and the change in entropy Delta V-r(m), for formation of the complexes at T = (278.15 to 393.15) K and m = (0 to 0.33) mol . kg(-1). These results are compared with those recently obtained for the aqueous complexes of 18-crown-6 with K+, Na+, and Ba2+. (C) 2005 Elsevier Ltd. All rights reserved.