Small amounts of water have previously been found to have a dramatic effect on the value of the seismic dissipation factor Q-1. This suggests the importance of controlling the environment in which Q-1 measurements are made. Since the effect of moisture is so large, studying the variation in Q-1 with partial pressure of water and other volatiles can provide important clues for understanding the attenuation mechanisms in 'room dry' rocks. Thus measurements have been made of both Q-1 and velocity for several limestones (all >99% CaCO3) and sandstones. The Q-1 of both limestones and sandstones is affected by the presence of water vapor, but at any given partial pressure of water (PH2O) the Q-1 of the limestones is lower than the Q-1 of the sandstones. Velocities of the sandstones decreased by as much as 30% as the relative humidity was changed from 0 to 80%. Velocities of limestones of comparable porosity decreased only 2% over the same range. Simultaneously with the Q-1 and velocity measurements, adsorption isotherms (~23¿C) were measured on a companion sample. Generally, limestones adsorbed less water at a given partial pressure than the sandstones. The amount of water adsorbed, at most 2-3 mg/g, does not increase the density enough to account for the decrease in velocity, particularly in the sandstones. The Brunauer-Emmett-Teller equation was used to calculate surface areas from the adsorption isotherms. Surface areas ranged from 0.5 to 3.4 m2/g. Limestones had lower surface areas than the sand stones. The larger surface areas in the sandstones can probably be attributed to varying amounts of clays. A plot of Q-1 versus mass adsorbed shows that most of the increase in dissipation is associated with the adsorption of the first two monolayers of water. A fixed amount of water causes a greater change in the Q-1 of the sandstones than of the limestones. One sandstone was measured by using anhydrous benzene as the adsorbate. In contrast to the action of the water, Q-1 changed very little with increased benzene pressure for small P/P0, even though significant amounts of benzene were adsorbed. For P/P0 greater than 0.7, there were significant increases in Q-1. It appears that there are two mechanisms of attenuation in 'dry' rocks: one is observed at low P/P0 and is associated with surface effects that are strongly dependent on the nature of the adsorbate, and the other is observed at high P/P0 and is associated with fluid flow in fine pores.