In the presence of 18-crown-6, D2O forms a 1:1 complex with the macrocyclic molecule in supercritical fluid CO2 with two different configurations. The D2O molecule can be bonded to two oxygen atoms of the crown cavity in a bridged configuration that is characterized by a broad peak at 2590 cm(-1). The D2O molecule can also form one hydrogen bond with an oxygen atom of the crown cavity that can be characterized by two peaks at 2679 and 2733 cm(-1), with the former assigned to the hydrogen-bonded O-D stretching and the latter the unbonded O-D stretching. The equilibrium constants of the two configurations in supercritical CO2 have been calculated. The enthalpy of formation is -12 +/- 2 kJ mol(-1) for the single-hydrogen-bond complex and -38 +/- 3 kJ mol(-1) for the bridged configuration complex. At high 18-crown-6 to D2O ratios, the formation of another complex in supercritical CO2 that involves one D2O molecule hydrogen bonded to two 18-crown-6 molecules becomes possible.