Massive pyrite was shown to produce soluble iron, hydrogen, and sulfate ions on exposure to air and water. The rate of this process was directly proportional to the surface area of the mineral; it was unaffected by a drop in the pH and the presence of the ferrous and sulfate ions formed. Cupic ion had no effect but ferric ion accelerated pyrite degradation until all the ferric ion was consumed, in accordance with FeS2+ 2Fe3+—>‐3Fe2++ 2S°.Thiobacillus ferrooxidansincreased pyrite degradation considerably; the presence ofThiobacillus thiooxidanshad no influence on pyrite degradation.