The relative concentrations of Cl, Br, I, and K in fluid inclusions in hydrothermal minerals were measured by laser microprobe noble gas mass spectrometry on irradiated samples containing 10⁻¹⁰ to 10⁻⁸ L of fluid. Distinctive halogen signatures indicate contrasting sources of fluid salinity in fluid inclusions from representative “magmatic” (St. Austell), “metamorphic” (Alleghany), and “geothermal” (Creede, Salton Sea) aqueous systems. Br/Cl mol ratios are lowest at Salton Sea (0.27–0.33 × 10⁻³), where high salinities are largely due to halite dissolution; intermediate at St. Austell (0.85 × 10⁻³), possibly representative of magmatic volatiles; and highest (near that of seawater) at Creede (1.5–2.1 × 10⁻³) and Alleghany (1.2–2.4 × 10⁻³), where dissolved halogens probably were leached from volcanic and (or) nonevaporitic sedimentary rocks. IC1 mol ratios are lowest (near that of seawater) at Creede (1–14 × 10⁻⁶), possibly because organisms scavenged I during low temperature recharge; intermediate at Salton Sea (24–26 × 10⁻⁶) and St. Austell (81× 10⁻⁶); and highest at Alleghany (320–940 × 10⁻⁶), probably because the fluids interacted with organic-rich sediments at high temperatures before being trapped. KCl mol ratios indicate disequilibrium with respect to hypothetical feldspathic alkali-Al-silicate mineral buffers at fluid inclusion trapping temperatures at Creede, and large contributions of (Na, K)-bicarbonate to total fluid ionic strength at Alleghany. Significant variations in Cl/Br/I/K ratios among different fluid inclusion types are correlated with previously documented mineralization stages at Creede, and with the apparent oxidation state of dissolved carbon at Alleghany. The new data indicate that Cl/ Br/I ratios in hydrothermal fluid inclusions vary by several orders of magnitude, as they do in modern surface and ground waters. This study demonstrates that halogen signatures of fluid inclusions determined by microanalysis yield important information about sources of fluid salinity and provide excellent definition of fluid reservoirs and tracers of flow and interaction in ancient hydrothermal systems.