The Hg isotopic compositions of samples throughout the vertical extent of two fossil hydrothermal systems were analyzed by multicollector inductively coupled plasma–mass spectrometry. Results show >5‰ (δ202Hg/198Hg; relative to NIST 3133) fractionation, more than 50 times greater than the 0.1‰ (2σ) external reproducibility of the analyses. The Hg isotope compositions from both hydrothermal systems can be grouped by dominant mineralogy and position; δ202Hg/198Hg values at the tops of the systems are −3.5‰ to −0.4‰ in cinnabar-dominant sinter and −0.2‰ to +2.1‰ in metacinnabar-dominant sinter, and the underlying veins have δ202Hg/198Hg values of −1.4‰ to +1.3‰. These differences probably resulted from the combination of boiling of the hydrothermal fluid, oxidation near the surface, and kinetic effects associated with mineral precipitation. The natural variation in Hg isotopic compositions observed in this study is higher than that expected from the trend of decreasing mass-dependent fractionation with increasing mass extrapolated from stable isotope systems up to Z = 26 (Fe), confirming that even the heaviest elements undergo significant stable isotope fractionation in hydrothermal systems.