Oxidation reactions for S, As, and Fe have been measured downstream in several hot spring discharges at Yellowstone National Park. Oxidation of dissolved H₂S to S₂0₃ is the fastest reaction and occurs abiotically and nearly instantaneously on exposure to air. In high pH overflows (pH> 7), S₂O₃ shows little tendency to oxidize further. In low pH overflows, S₂O₃ disproportionates, producing elemental sulfur and sulfite. Solid sulfur is then oxidized more slowly to sulfuric acid by microbial catalysis. Oxidation of dissolved As^III to As^V and F^II to F^III in acid waters (pH = 2.7) requires microbial catalysis and proceeds at rates comparable to those in acid mine drainage. The oxidation rate for Fe^II is about 2mmol/L/h and for As^III is about 0.07 mmol/L/h, both of which are many orders of magnitude faster than the abiotic rate.