Abstract During Ocean Drilling Program Legs 170 and 171A, logging-while-drilling (LWD) tools were deployed on the Costa Rica and Barbados subduction margins. High-quality density, resistivity, and natural γ-ray logs were acquired across the décollement zones on both margins. Based on a new method of ‘layer-by-layer’ correlation of the logs, changes in thickness and volume between incoming and subducted or accreted sediments are determined with 15 m resolution and 1% accuracy. The change in sediment thickness and volume generally decreases with depth, however, this change strongly depends on the lithology. Siliceous layers such as diatomaceous and radiolarian clay tend to be fluid-bearing, and the stratigraphic position of such zones is a critical factor in the fate of the subducted sediment section. On the Costa Rica Margin, the sediment section on the Cocos plate is underthrust intact beneath the toe of the Caribbean Plate with no frontal offscraping where a siliceous fluid-bearing zone is present only in the upper part of the section. On the Barbados margin, a layer of radiolarian clay exists, providing a narrow zone of mechanical weakness and anomalously high dewatering in the middle of the sediment section. This layer divides the sediments that are subducted from those that are accreted. Accreted and subducted sediments show different compaction styles. Accreted sediments are characterized by rapid compaction with vertical thickening, whereas subducted sediments are characterized by slow compaction with vertical flattening. The vertical thickening of the accreted sediments is due to horizontal tectonic compaction and contributes to the vertical thickening of the accretionary prism as a whole in the early stages of deformation. Dewatering flux is calculated by the volume change of the sediment sequences across the trench. The dewatering flux computed from the LWD data provides an estimate of the minimum fluid flux in the subduction zone and is significantly greater than the flux estimated from laboratory experiments because of meter-scale fluid conduits which influence the downhole logs but not the centimeter-scale sample measurements.