The velocity and absorption of ultrasonic waves in oxygen were measured by means of an improved ultrasonic interferometer in the range from one to one hundred Mc per atmosphere. Dispersion of the velocity ranged from 333.14 m/sec, to 357.22 m/sec at 30°C. The ratio (αexper/αclass) dropped from 3.68 to 2.05, and the corresponding value ofCv/Rdropped from 2.50 to 1.61. The increase in velocity and the decrease in (αexper/αclass) is interpreted as caused by the slow exchange of energy between the translational and rotational degrees of freedom. The relaxation time for the rotation was 5.24×10−9second.