Experimental measurements of the absorption of sound in still air exhibit a distinct deviation from the theoretical predictions based on known classical and molecular absorption losses. Absorption coefficients have been measured in humid air over the humidity range of 0% to 100% relative humidity. These measurements show the same type of low-frequency-absorption anomaly as has been found previously, but to a lesser extent. It is thought that the anomaly is due to the vibrational relaxation of nitrogen catalized by either water vapor, carbon dioxide, or a combination of both. Experimental sound-absorption measurements have also been made in humid nitrogen at room temperature. The data from these measurements, while quite scattered, do show a definite shift of the absorption peak with water-vapor content. These measurements also show fair agreement with values deduced from air-absorption measurements. [This work was supported by the U. S. Army Research Office—Durham, under contract.]