Carbon, oxygen, and hydrogen isotope results from carbonate and silicate fractions of altered core samples from the Yaxcopoil-1 borehole drilled into the 65 Ma Chicxulub impact crater provide constraints on the physico-chemical parameters of the hydrothermal solutions, and their likely origin. Yaxcopoil-1 impactites were initially permeated with calcite and halite at ambient temperature. This was followed by thermal metamorphism (diopside after igneous augite) and widespread Na-K metasomatism (feldspar after igneous plagioclase), which were overprinted by abundant lower-temperature clay and calcite. Silicate fraction isotopic values have δ 18 O SMOW values between 10 and 23‰ indicating important isotopic exchange between impact melt (∼8‰) and Cretaceous limestone (∼26‰). Heavier δ 18 O values occur over depth intervals with intense feldspar alteration (813–833 m and 864–872 m). The δD SMOW values (−34 to −54‰) are chiefly influenced by smectite abundance and roughly mirror δ 18 O values. Carbonate fraction δ 18 O SMOW values (22–30‰) are controlled by calcite contents, and several exceed the limestone signature. Most δ 13 C PDB (−1 to +2‰) values also cluster around that of local limestone, but a number are significantly lighter (down to −7‰). Isotopic and fluid inclusion results indicate hydrothermal fluid temperatures between 270 and 100 °C, high salinities (∼20%), and minor kerogen contents. These data are compatible with mineralogical constraints, which further support an increase in oxidation state with decreasing temperature. Isotopic data point to a saline CO 2 -bearing fluid mixed with small amounts of reduced carbon, and decarbonation and infiltration processes. Combined results are most consistent with a basinal oilfield saline brine that was driven by impact-induced heat.