The frequency dependence of seismic attenuation in a suite of clay-rich reservoir sandstones was investigated in the laboratory. Compressional- and shear-wave velocities (V P and V S ) and quality factors (Q P and Q S ) were measured as functions of pore-fluid viscosity at an effective pressure of 50 MPa and at an experimental frequency of about 0.8 MHz using the pulse-echo technique. The experimental viscosity ranged from 0.3 to 1000 centipoise, which gives equivalent frequencies for a water-saturated sandstone of 2.6 MHz to 780 Hz, assuming a global-flow loss mechanism. Two types of behavior were observed: high permeability (greater than 100 millidarcies) sandstones tend to show variable Q P and Q S which are similar in magnitude to those predicted by the Biot theory over the viscosity range 0.3 to about 20 centipoise (equivalent frequency range 2.6 MHz to about 39 kHz); low permeability (less than 50 millidarcies) sandstones tend to show almost constant Q P and Q S over the experimental viscosity range that are not predicted by the Biot theory. The Biot theory does not predict the observed Q P and Q S values in the high permeability sandstones for viscosities greater than about 20 centipoise, where the observed Q P values are generally much lower than the Biot predicted values. High permeability sandstones show small velocity dispersions with changing pore-fluid viscosity that are consistent with the Biot theory. Low permeability sandstones show relatively large increases in velocity with increasing viscosity not explained by the Biot theory, which are consistent with a local flow loss mechanism. The results indicate the presence of two dominant loss mechanisms: global flow (at least down to about 39 kHz in water-saturated rocks) in high permeability sandstones with only small amounts of intrapore clay, and local flow at ultrasonic frequencies in low permeability, clay-rich sandstones.