The attenuation of sound waves in mixtures of oxygen and water vapor contained in a tube has been measured at frequencies between 90 and 1200 cps. The tube was terminated at one end by a fixed reflector and at the other by a plane diaphragm producing the sound waves. The impedance of the diaphragm/gas‐column system was investigated near natural modes of the tube and analyzed by means of a circle diagram to yield the attenuation coefficient. The frequency of maximum absorption of sound was found to be a quadratic function of the water‐vapor concentration. The value of the vibrational specific heat for oxygen was found to be (0.0332+0.0015) R.