Ultrasonic absorption measurements were made to compare the relative effectiveness ofH2OandD2Omolecules in influencing thermal vibrational relaxation inN2Omolecules. The displacement rates of the absorption per wavelength maxima induced byH2OandD2Owere compared inN2O ‐ H2OandN2O ‐ D2Omixtures. Experimental apparatus was constructed for these measurements which utilized the “direct” absorption technique in conjunction with pulsed sound waves, having a frequency of approximately 113 kc. The parameter of frequency/pressure was varied by changing the gas pressure.The data were taken at a temperature of 21°C in the frequency/pressure range of 100–700 kc/atmos. The maximum amplitude of the molecular absorption coefficient per wavelength in dryN2Owas 0.153 and this occurred at 210 kc/atmos. Measurements were taken inN2O ‐ H2OandN2O ‐ D2Omixtures atH2O‐vapor andD2O‐vapor concentrations of 0.39%, 0.77%, and 1.14%.The usual single relaxation time theoretical absorption curve could be fitted to the data for driedN2O. However, in the mixtures there was some evidence of two separate relaxation times, one possibly associated withN2O ‐ N2Ocollisions and the other withN2O ‐ H2OorN2O ‐ D2Ocollisions. The maximum absorption coefficient in the mixtures was somewhat less than that observed in the driedN2O. TheH2Omolecules were 1.72 times more effective than theD2Omolecules in shifting the frequency associated with the absorption maximum.