Sound absorption and velocity measurements have been made inCO2/D2Omixtures at 300°, 400°, and 500°K. The results show thatD2Ois from13to15as efficient asH2Oin de‐exciting the bending‐mode vibration ofCO2in a molecular collision. The collision efficiency ofD2Ofor de‐excitingCO2decreases with temperature at an even greater rate than does that ofH2O. The large difference betweenD2OandH2Oinvalidates the chemical affinity explanation of effect ofH2OonCO2. An approximate semiclassical calculation of the transition probability indicates that it is possible to attribute the relaxation to an interchange between the vibration of theCO2and the rotation of theH2OorD2O.