A conductance study of the interaction between TI+, Ag+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, and Pb2+ ions and aza-18-crown-6 in dimethylformamide solution has been carried out at various temperatures The formation constants of the resulting 1:1 complexes were determined from the molar conductance-mole ratio data and found to vary in the order TI+ > Ag+ > Pb2+ > Cd2+ > Cu2+ > Ni2+ Co2+ > Zn2+. The enthalpy and entropy of complexation reactions were determined from the temperature dependence of the formation constants. In the ease of all M2+-A18C6 systems studied, the resulting complexes are both enthalpy and entropy stabilized While, in the ease of Ag+ and TIC ions, the resulting complexes with A18C6 are enthalpy stablizied but entropy destabilized.