The ultrasonic properties of dry,CO2‐free air have been measured at 32°C andf/pvalues ranging from 2 to 125 Mc/atmos. Dispersion of the velocity has been found beginning at 30 Mc/atmos, accompanied by a large increase in absorption such that measurements could not be made above 130 Mc/atmos with the equipment used. Changes in velocity, absorption, and specific heat are interpreted as owing to the slowing of energy exchange between translational and rotational states. The relaxation time derived from the dispersion measurements has been found to be 2.29 × 10−9sec, corresponding to a midpoint of the dispersion curve at 116 Mc/atmos.