Forty-one interstitial water samples recovered from ODP Site 1202, Okinawa Trough, were analyzed for major constituents, B and δ¹¹B. The geochemical results show that major constituents and boron content varied largely in pore fluids and possibly were affected by sulfate reduction, recrystallization of biogenic carbonate or silica, ash alteration, and organic matter degradation. Mixing of fluids along high-porosity sandy layers or fracture zones also changes the pore water chemical compositions significantly.