Isotopic analyses were made on whole-rock samples and minerals from 25 granites, 21 rhyolites, and 10 basaltic dikes and sills from the l,500-m.y.-old St. Francois Mountains terrane. The δD of chlorite is relatively uniform at −44 to −65, but the whole-rock and feldspar δ18O values systematically increase from +7 in the northeast to +14 in the southwest, correlating with an increasing intensity of “brick-red” alteration of the K-feldspars. Coexisting quartz and feldspar are typically not in isotopic equilibrium, except in the northeast. The coarser grained (>1 mm) quartz (δ18O = 8.8 to 10.6) is isotopically similar to “normal” igneous quartz, but the δ18O of the finer grained quartz correlates with the whole-rock δ18O pattern. Although the St. Francois terrane is geologically similar to many low-18O Tertiary volcanic-plutonic complexes that have interacted with meteoric-hydrothermal fluids at high temperatures, only a single locality (Stono Mountain) contains low-18O quartz and feldspar (δ18O quartz = 4.6 to 5.4). Even at this locality, the feldspars were later enriched in 18O by the same alteration event that affected the rest of the region. This second stage of hydrothermal activity apparently involved aqueous fluids having δ18O ≈ 0 to −6 and δD = 0 to −25. The temperature of alteration may have been as low as 50° to 100°C in the southwest (upper part of the volcanic section). This event affected some of the basaltic dikes and sills, but others were not altered and must have been intruded afterward. Rb-Sr and K-Ar age data suggest that the regional alteration occurred as late as 1,100 to 1,200 m.y. ago, long after the age of primary igneous activity. The hydrothermal fluids apparently originated as meteoric surface waters, indicating that during late Precambrian time such waters were isotopically similar to present-day meteoric waters in warm climates; this interpretation implies that the late Precambrian oceans must have been isotopically similar to present-day ocean.