Drilling at Soultz-sous-Fore⁁ts, France, conducted as part of the European Hot Dry Rock Project, intersected a fracture lined with vein quartz and actively producing hot (150 ¿C) water at 2174 m depth in the granite basement to the Rhine Graben. At least seven generations of quartz are present within the vein, but fluid inclusion homogenization temperatures show that temperatures remained within 15 ¿C of the present field temperature throughout its development, apart from a few rare pulses of hotter water. In contrast, freezing measurements indicate two distinct fluids. The more saline one, ranging from 10 to 15 wt % NaCl eq, dominated the early stages of vein fill, while the second, ranging from 0 to 8 wt % NaCl eq, become more prominent in later stages. Oxygen isotope analyses of different quartz generations also show two populations in addition to magmatic relicts. In the early stages a population with Δ18O from 14? to 18? predominates and is consistent with growth from a sedimentary basin brine. Later quartz shows lighter values (12?--13?), resulting from precipitation from a fluid isotopically identical to the modern fluid, which is in equilibrium with granite feldspar at a temperature ~25 ¿C higher than the present vein temperature. The change in Δ18O does not, however, correlate with that in fluid salinity, reflecting the different effects of fluid-rock interaction on the two parameters. Changes in fluid salinity, possibly resulting from meteoric inputs into the deep formation waters of the Rhine Graben, occurred early in the flow path of the Soultz fluids because there is no decrease in fluid temperature associated with even the most dilute inclusion fluids. With time the path by which these fluids from deep in the Mesozoic sediments of the graben have made their way to the Soultz site has changed, resulting in greater interaction with granite, as recorded in the oxygen signature. ¿ 1998 American Geophysical Union