Oxygen-isotopic composition data are reported for young oceanic basalts from two different tectonic settings in the eastern Pacific: (1) the uppermost basement on the Galapagos Rift (< 1 Ma old), and (2) subbasement depths up to ≈150 m in the Gulf of California (≤ 3.5 Ma old). In the Galapagos area, whole-rock δ18O values exhibit a narrow range of 5.7–6.6‰, consistent with minimal to slight low-temperature alteration. Limited alteration probably reflects a combination of young basement age, low temperatures for solutions in the uppermost basement, and confinement of solutions mainly to discrete fractures. In the Gulf of California, by contrast, high whole-rock values are found at site 474A, near the Baja margin, where the greatest range in δ18O also occurs, from 2.5 to 12.5‰. This indicates considerable variation in both the temperature and degree of alteration at this site, probably a consequence of the heterogeneous basement sequence (intercalated pillow lavas, sills, and wet sediments). Differences in δ18O of up to 6‰ can exist over several metres, implying sharp changes in temperature or local water/rock ratio. The development of low-temperature alteration is more pronounced at site 474A (≈3.5 Ma) than at the several other (younger) crustal locations drilled in the Gulf of California, presumably because of longer exposure to seawater during recession of site 474A from the spreading axis.Analysis of primary magmatic phases at all sites yield 18O fractionations between plagioclase, clinopyroxene, and magnetite consistent with crystallization in a closed system and without high-temperature subsolidus reaction with seawater oxygen (even where sediments are intercalated within the basement). At site 485 in the Gulf of California, a thick basaltic sill yielded the lowest calculated crystallization temperatures (920 ± 50 °C) on the basis of plagioclase–magnetite fractionations; this agrees reasonably with groundmass quartz estimated to have formed at 860 ± 40 °C. Two vein carbonates from the uppermost part of the igneous sequence at this site were formed at temperatures of less than a few tens of degrees.We also report high-precision strontium- and oxygen-isotopic data on mineral separates from four basalts recovered at site 504B (6 Ma old). The most striking feature of these data is very substantial contamination of most mineral phases by seawater Sr, even though the same splits have typical primary magmatic oxygen-isotopic values. Relative to an unaltered mid-ocean ridge basalt 87Sr/86Sr value of 0.70265 ± 0.0001, 87Sr/86Sr ratios of plagioclase range from 0.70270 to 0.70359 (for apparently primary δ18O values of 5.35–5.75‰); for clinopyroxene, 87Sr/86Sr ratios range from 0.70309 to a very enriched value of 0.70695 (δ18O values of 4.97–5.30‰, with one value of 6.05‰). These results indicate that young basalts may have experienced significant interaction with seawater, which is not recorded in terms of oxygen-isotope exchange.