Elastic wave velocities and the quality factor Q were measured on specimens of Berea Sandstone as their water vapor content was increased by exposure to an environment of increasing relative humidity (RH). At a maximum value of RH (= 0.98), the moisture absorbed was about 0.15% (vs. 9.62% for full saturation) of their dry mass.The bar velocities in the torsional and longitudinal vibrational modes were measured in the frequency range 1–20 kHz, while the pulse first-arrival compressional and shear-wave velocities were obtained at 820 kHz. Over the range of variation in relative humidity from 0.0 (vacuum ~1 Torr) to 0.98 the velocities showed a decrease of 20–30%, the bar velocities undergoing the greater change. Over the same range the Poisson's ratio fell from 0.2 to 0.1.The value of the quality factor Q fell sharply from ~200 to ~50 during the initial increase (about 0.01% by weight) in the moisture content. Further addition of water vapor produced no significant change in Q for either mode. Also, no systematic variation of Q with frequency was observed. However, when the amount of moisture in the specimens increased to about 0.15% by weight, not only did the Q values decrease but also began to show a frequency dependence. This change in the frequency variation of Q suggest that, while the water molecule layers initially absorbed by the specimens form a solid phase, a thickness is reached beyond which they depict bulk water behavior. A rough estimate of this thickness was found to be about 110 Å.On a mesuré la vitesse des ondes élastiques et le facteur de qualité Q sur des spécimens de grès de Berea lorsqu'on augmentait leur teneur en vapeur d'eau par exposition à un milieu d'humidité relative (HR) croissante. Pour la valeur maximale de HR (= 0.98), l'humidité absorbée était d'environ 0.15% (par rapport à 9.62% pour la saturation complète) de la masse à l'état sec.