New measurements of seismic velocity and attenuation have been made in the kiloHertz frequency range to temperatures of 120°C, confining and pore pressure to 200 bars in Berea sandstone. With increasing temperature, shear velocity and attenuation decrease at all pressures in a fully saturated rock. The data suggest that thermal relaxation is not a significant loss mechanism under these conditions. We propose that dissipation is controlled by a viscous fluid flow mechanism, in which a sharp frequency peak in attenuation is shifted from 2kHz at room temperature to 8 kHz at 120°C as the pore fluid viscosity is decreased with increasing temperature. However, the velocity decrease is too great to be accounted for by a change of relaxation times. A non‐dispersive temperature softening in shear may control the velocities.