Dissolved boron in seawater from the Atlantic and Pacific is isotopically homogeneous at 39.5 per mil ( 11B /10B ratios are expressed as per mil deviations from NBS SRM 951). Unaltered mid-ocean ridge basalts (MORE) from the crest of the East Pacific Rise (EPR) at 21° and 13°N have B contents of 0.39 ± 0.03 and 0.46 ± 0.03 ppm (about one order of magnitude lower than previous estimates) and δ11B of -3.6 ± 0.4 and -2.2 ± 0.6 per mil respectively. Large scale B exchange between seawater and the oceanic crust has been demonstrated at both high and low temperature. Hydrothermal solutions from nine separate vent fields at 21° and 13°N (EPR) have variable B enrichments, relative to seawater (416 μmoles/kg), of between 8 and 32% and have δ11B values between 30.0 ± 0.4 and 36.8 ± 0.4 per mil. Boron has been extracted from the basalts with no resolvable isotopic fractionation. High temperature water/rock ratios, based on the B concentrations, are between 0.28 and 3.0. The hydrothermal flux of B into the oceans is between 0.4 and 0.8 × 10 9 moles/yr, assuming that only pristine basalts are present in the reaction zone. Basalts altered at low temperature and serpentinites are variably enriched in B. The B content of altered whole rocks correlates strongly with δ 18O, and increases with degree of alteration. Altered basalts ( n = 7) containing between 8.9 and 69 ppm B have δ11B between 0.1 and 9.2 ± 0.4 per mil. Model calculations give water/rock ratios greater than 100 for rocks recovered from DSDP Hole 418A. Serpentinized peridotites ( n = 4) with between 50 and 81 ppm B have δ11B between 8.3 ± 0.4 and 12.6 ± 0.4 per mil. The flux of B into the crust during low temperature alteration is about 13 × 10 9 moles/yr. The maximum diffusive flux of B into the crust from sediment pore waters is 0.8 × 10 9 moles/yr with a δ11B less than 43 per mil.