The product distributions and mechanisms of FeS hydrolysis were examined as a function of pH, NH/sub 4//sup / ion concentration, and SO/sub 2//sulfite concentrations in aqueous suspensions. Products in the solution phase as well as precipitates and starting material residues were examined by pH and redox potentiometry, x-ray diffraction, x-ray fluorescence, scanning electron microscopy, infrared spectrometry, and wet chemical methods. Principal products of pyrrhotite hydrolysis included soluble SO/sub 4//sup =/, Fe/sup 2 /, and lepidocrocite (FeOOH). In an unbuffered acid medium, equimolar leaching of iron and sulfur from FeS gives these plus some evolved H/sub 2/S as the only products. Hydrolysis in unbuffered water yields the same products with the exception of H/sub 2/S, with an atom ratio of Fe:S leached of 3.5 to 1. Surface pyrite (FeS/sub 2/), S/sup 0/, and S-rich sulfides are not observed. Hydrolysis in ammonia at pH = 9 gives rise to equimolar FeS leaching to form Fe(OH)/sub 2/, FeO(OH), and SO/sub 4//sup =/ as the only products. Hydrolysis in a bisulfite buffer (pH = 6) gives Fe/sup 2 / plus lesser amounts of FeO(OH), FeO, and Fe/sub 2/(SO/sub 4/)/sub 3/. Thermodynamic and kinetic considerations, extended to present conditions, predict FeO(OH), Fe/sup 2 / and SO/sub 4//sup =/ as the principal products. The kinetics of iron sulfide hydrolysis are exceedingly slow. In no case did more than 2 percent of the original FeS leach even after 6 days.