The hydrostatic pressure dependences of the velocities of longitudinal and shear ultrasonic waves propagated in amorphous selenium have been measured as a function of temperature through the glass-transition region. Both δC11,δP and δC44/δP increase rapidly as temperature is increased, a finding attributed to a combination of increasing vibrational anharmonicity as Tgis approached from below and to free-volume effects. At room temperature, substantially below Tg. the thermal Grüncisen parameter 7this only 1·05 whereas the mean long-wavelength acoustic parameter 7elis 3·0. This arises because the intermolecular volume is much more compressible than the intramolecular volume: the Gruneisen parameters for purely internal modes of vibration are small whereas those for the external modes are larger.