The oxygen isotopic compositions of pore fluids from sediments of older oceanic crust commonly decrease with depth. Increases in calcium ion concentrations always accompany the 18O/16O decreases. These changes are attributed principally to the alteration of basalts or alteration of volcanic ash in the sediments. Oxygen iostope material balance calculations suggest that 5--20% of the upper kilometer of basalt is altered at the time of subduction of the oceanic crust. This corresponds to a flux of 18O into the oceanic crust of 2.3¿1015 moles per million years. Evaluation of the oxygen and hydrogen isotope data from the pore fluids also suggests that the principal process of mass transfer in the sediments of older oceanic crust is diffusion. Convention of water in the basalts of the oceanic crust appears to continue until subduction.