Results from a comprehensive study of B, δ11B, Li, and δ6Li distributions in both sediment and pore-water samples of Ocean Drilling Program Site 808, Nankai Trough, Japan, show a strong correlation between Li and B concentrations, in agreement with previously published information from surface-marine sediments, suggesting similar geochemical behavior in sedimentary environments. The δ6Li and δ11B analyses provide additional data on the systematics of Li and B in marine sediments. Our results provide information on fluid-expulsion activity that has occurred at this drill site. Anomalies in all these constituents in the vicinity of the decollement zone (∼960 m below sea floor [m bsf]) suggest an influx of deeply generated fluids having distinct chemical compositions. The maximum concentrations of B and Li in bulk sediment, as well as their corresponding isotopic compositions, suggest uptake of both elements via chemisorption or secondary-mineral formation near the fluid conduit associated with the decollement zone. The low δ11B in pore waters suggests contribution of exchangeable plus lattice-bound B from greater depths. The corresponding high δ6Li similarly indicates mobilization from sediment under high-temperature conditions.