The seismic refraction data obtained from 48 sonobuoy stations on the Sunda shelf provide details of the sediment velocity structure down to depths of 8 km (26,200 ft). In addition, velocity and depth information were determined in places for the upper crystalline crust down to 11 km (36,100 ft). The steep velocity gradients that characterize the West Natuna basin sediments are attributed to low sedimentation rates. Pronounced changes in the sediment velocity structure within disturbed sediments on the western edge of the Sarawak basin support the claim that the subduction melange (accretionary prism) on Borneo extends into the Sarawak basin. Data also indicate that the zone of thickened subduction melange sediments may extend northward to the shelf edge. New crustal stru ture data show an important unreported basement salient in the eastern part of the West Natuna basin, and require a shift of approximately 45 km (28 mi) in the previously reported western boundary of the Cretaceous subduction melange. It has been suggested previously, and our data confirm, that oceanic crust, which had been subducted prior to the beginning of the Oligocene, is now covered by 8 km (26,200 ft) of undisturbed sediment in the Sarawak basin. New seismic data provide strong evidence that the crust beneath the Sarawak basin is oceanic, implying that the shelf edge has advanced about 300 km (185 mi) northward over oceanic crust as a result of the progradation of post-Eocene sediments. Except for the subduction melange, pre-Oligocene sediments are quite thin in the Sarawak basin. /P>