Thiosulfate (S₂O₃^2-), polythionate (S_xO₆^2-), dissolved sulfide (H₂S), and sulfate (SO₄^2-) concentrations in 39 alkaline and acidic springs in Yellowstone National Park were determined. The analyses were conducted on site, using ion chromatography for thiosulfate, polythionate, and sulfate, and using colorimetry for dissolved sulfide. Thiosulfate concentrations ranged from <0.01 to 8 ppm. Polythionate was detected only in Cinder Pool, Norris Geyser basin, at concentrations up to 1.8 ppm, with an average sulfur-chain-length from 4.1 to 4.9 sulfur atoms. The results indicate that no thiosulfate occurs in the deeper parts of the hydrothermal system. Thiosulfate may form from (1) hydroIysis of native sulfur by hydrothermal solutions in the shallower parts (<50m) of the system, (2) oxidation of dissolved sulfide upon mixing of a deep hydrothermal water with aerated shallow groundwater, and (3) the oxidation of dissolved sulfide by air. Air oxidation of H₂S-containing hot springs proceeds very rapidly.