During subduction, hydrothermally altered oceanic crust undergoes extensive dehydration. The water released migrates upwards and may escape along the slab-mantle interface, or continue rising to hydrate the mantle overlying the subducting slab. Oxygen isotope ratios from eclogitic ophiolites of the western Alps correlate closely with lithology. Samples with little-altered basalt as a protolith have δ18O values of 5.1-6.0‰. Material representing low-temperature alteration zone rocks have δ18O values of 5.9-6.6‰ and rocks orginating in high-temperature alteration zones have δ18O values of 4.6-5.3‰. These isotopic values closely approach those of the ocean crust and unmetamorphosed ophiolites. These data suggest that the Alpine rocks have not experienced significant fluid flow during metamorphism, implying that the escape of fluid from subducted ocean crust is highly focused, probably along structurally controlled pathways to the top of the slab. The fluid may then continue largely along the slab-mantle interface, although a portion of it rises further to hydrate the overlying mantle.