Comparison of shipboard acoustic velocity measurements from Leg 149 with velocities determined from seismic experiments shows that the shipboard data are poor indicators of in situ velocity in the sedimentary section beneath the Iberia Abyssal Plain. Experiments conducted under pressure yield estimated in situ velocities that agree well with seismic surveys on the Iberia Abyssal Plain. The laboratory data indicate a modest increase in velocity in the upper 335 mbsf, from about 1800 m s-1 at the seafloor to about 2000 m s-1 at 335 mbsf. A steep velocity gradient is interpreted in the interval between 335 and 370 mbsf, and it may be the cause of a reflection observed in the seismic data. An increase in effective pressure below about 200 mbsf is interpreted to represent a transition from poorly consolidated sediments to deeper consolidated sediments. Contrary to the shipboard acoustic velocity measurements, no evidence is seen for major differences in the velocity gradient in the upper 500 mbsf between Site 900 and Sites 897, 898, and 899. The steeper velocity gradient observed in the shipboard data from Site 900 is attributed to more pronounced decompression of the samples following recovery, which implies a difference between the geotechnical properties of the silty clays recovered at Site 900 and the other sites. The in situ porosity and density inferred from the laboratory experiments follow expected theoretical trends, indicating an exponential decrease in porosity and linear increase in density with depth.