The acoustic properties of deep-sea claystones and silty claystones recovered on Ocean Drilling Program Leg 149 were examined under pressure conditions at and above the expected in situ effective pressure in order to estimate acoustic velocity in the upper few hundred meters below seafloor (mbsf) on the Iberia Abyssal Plain. In situ effective pressure and velocity for each of the samples studied is indicated by an inflection point on the measured velocity vs. pressure curve. The estimated in situ velocities range from 1995 to 2470 m s (super -1) for sediments buried from 120 to 510 mbsf, and the estimated in situ effective pressure ranges from 2.2 to 6.5 MPa. Velocities estimated from the laboratory study are similar to those determined from marine seismic-refraction surveys. The laboratory data suggest a velocity gradient of about 0.63 m s (super -1) at depths shallower than 320 mbsf and a velocity gradient of 4.4 m s (super -1) /m between 335 and 370 mbsf. The steep velocity gradient in the interval between 335 and 370 mbsf may be responsible for a strong refraction observed on marine seismic data at a depth of a few hundred meters. The in situ effective pressure increases abruptly below about 200 mbsf. This may reflect a change from poorly consolidated sediments above 200 mbsf to consolidated sediments below 200 mbsf. In situ pressure estimates combined with shipboard grain-density measurements provide an estimate of the porosity and density of the upper few hundred meters of sediment beneath the Iberia Abyssal Plain. The estimated porosity shows wide scatter, but generally appears to follow an exponential decrease in porosity with depth. Density generally appears to increase linearly with depth in the upper 600 mbsf according to the relation rho (z) = 1.7945+0.001 z, where z is depth below seafloor (in meters) and rho is density in grams per cubic centimeter.