Improvements to an ultrasonic interferometer are discussed. As a result a greater sensitivity to acoustic reactions is obtained and the reproducibility of the data is greatly improved. Velocity measurements in driedCO2‐free air, driedN2, and driedH2are given. Amplitude absorption coefficients forH2,N2, andCO2are also included. Measurements are made over the temperature range from 9°C to 36.6°C and over the pressure range from 82.17 cm Hg to 0.45 cm Hg. The frequencies extend from approximately 500 kc to 2.16 mc. A maximum value for the absorption inH2, attributed to molecular absorption, is located at anf/pratio of approximately 10 mc/atmos.