To understand better the seismic attenuation in the upper volcanic regions of the oceanic crust, compressional wave attenuations of oceanic basalts have been measured as a function of confining pressure using an ultrasonic pulse-echo spectral ratio technique capable of measuring attenuations to pressures of 500 MPa. Seven basalts, five from Deep Sea Drilling Project cores and two from dredge samples, have wide ranges of densities, porosities, and alterations, making possible an analysis of the parameters influencing basalt attenuation. Attenuation increases with the volume of secondary minerals present and with increasing porosity. Thus vesicularity and compositional changes associated with basalt alteration will produce variations in attenuation. With the application of hydrostatic pressure, cracks close, thereby reducing attenuations. This pressure dependence should be manifested in oceanic layer 2 by decreasing attenuation with depth. An inverse relationship between velocity and attenuation is observed at high hydrostatic pressures. Water saturation increases attenuation at pressures below 200 MPa and enhances the sensitivity of attenuation to pressure, thus making the state of saturation important in the 40 to 100 MPa range generally found in layer 2. These results provide a framework for interpreting marine attenuation data. ¿1990 American Geophysical Union