The Sm-Nd, Rb-Sr, and 18O/16O isotopic systems have been used to distinguish between the effects of seafloor hydrothermal alteration and primary magmatic isotopic variations. The Sm-Nd isotopic system is essentially unaffected by seawater alteration, while the Rb-Sr and 18O/16O systems are sensitive to hydrothermal interactions with seawater. Sm-Nd mineral isochrons from the cumulate gabbros of the Samail ophiolite have an initial 143Nd/144Nd ratio of &egr;Nd = 7.8¿0.3, which clearly substantiates the oceanic affinity of this complex. The initial 143Nd/144Nd ratios for the harzburgite, plagiogranite, sheeted diabase dikes, and basalt units have a limited range in &egr;Nd of from 7.5 to 8.6, indicating that all the lithologies have distinctive oceanic affinities, although there is also some evidence for small isotopic heterogeneities in the magma reservoirs. The Sm-Nd mineral isochrons give crystallization ages of 128¿20 m.y. and 150¿40 m.y. from Ibra and 100¿20 m.y. from Wadi Fizh, which is approximately 300 km NW of Ibra. These crystallization ages are interpreted as the time of formation of the oceanic crust. The 87Sr/86Sr initial ratios on the same rocks have an extremely large range of from 0.7030 to 0.7065 and the Δ18O values vary from 2.6 to 12.7. These large variations clearly demonstrate hydrothermal interaction of oceanic crust with seawater.