ODP Legs 129 and 185 sampled the upper 474 m of ∼170 Ma ocean crust in the western Pacific in order to investigate alteration processes in fast spread crust and to determine inputs to subduction. Fourteen composite bulk samples of altered upper oceanic crust from Site 801 have δ¹⁸O = 8.7–25.7‰, δD = −69.4‰ to −90.4‰, and δ¹³C = −2.7‰ to 1.8‰. The intensity of alteration and the amount of sediment within the basement decrease with depth, leading to corresponding decreases in δ¹⁸O and δD. A SUPER composite, constructed to estimate the bulk composition of the upper crust, has δ¹⁸O = 12.0‰, δD = −87.0‰, and δ¹³C = 0.7‰. Compared to core descriptions and geophysical logs, the SUPER composite contains too much ¹⁸O-rich sediment (δ¹⁸O = 25.7‰), and a corrected δ¹⁸O value of 10.8‰ is more reasonable for the upper crust at Site 801. These values are higher than those for other bulk upper oceanic basement sections (δ¹⁸O = 8.0–10.0‰) and result in part from: (1) intense low-temperature (<100°C) hydrothermal alteration of the upper 100 m of tholeiitic basement at Site 801 that may not be representative of material subducting in the western Pacific and (2) an aging effect, whereby progressive addition of ¹⁸O-rich secondary carbonate in veins and breccia cements contributed to the high bulk δ¹⁸O of this old upper crustal section.