Chemical and strontium isotopic analyses have been performed on 10 geothermal waters from the Valles caldera/Jemez Mountain region, as well as 10 rock samples collected in the VC-1 scientific core hole. The 87Sr/86Sr values of other rocks sampled in the Jemez Mountains have also been used in this study to understand relations between the fluids and rocks of the volcanically driven geothermal system and the lateral outflow plume, discharging down the Jemez fault zone. The strontium content of ground-water is linked to the host rock composition, the aquifer temperature, and therefore the solubility of Sr-containing minerals. Low Sr concentrations in the fluid are found with a high temperature and/or volcanic host rocks, whereas high Sr concentrations are characteristic of a low temperature and/or a Paloezoic sedimentary aquifer. Important variations of the 87Sr/86Sr value in hydrothermal waters (0.70842--0.72193) are related to chemical composition, Sr content, and host rocks. For VC-1 core samples, the Sr isotopic ratios are also markedly variable (0.70459--0.75686), low for the Quaternary volcanics and high for the Paleozoic sedimentary rocks. The same observation is made for other Jemez Mountains rocks (0.70465--0.8163) with, in addition, high 87Sr/86Sr values for Precambrian crystalline rocks. A good correspondence is generally found between the waters and the reservoir rocks studied here, which indicates that isotopic equilibrium is rapidly attained. The following suite of hydraulically related fluids displays a progressive enrichment in radiogenic strontium: Baca wells (87Sr--86Sr=0.7084--0.7094), VC-1 core hole at 483 m (0.7152--0.7154), Main Jemez Spring (0.7217), and Soda Dam (0.7219). This trend is associated with flow through Paleozoic rocks and with the length of the flow path from the center of Valles caldera (primary geothermal fluids) toward San Deigo Canyon, along the Jemez fault zone (derivative waters). ¿ American Geophysical Union 1988