Findings and conclusions from a 2-year study concerning chemical agents for the inhibition of acid mine drainage (AMD) formation are reported. Three materials containing pyrite were evaluated in detail, but only one was a strong producer of AMD. Analysis of the experimental results strongly suggests that the rate at which pyrite is oxidized can be lowered substantially if the iron products from pyrite oxidation are kept insoluble. Analysis of the data does not distinguish between the following mechanisms: the lack of availability of soluble ferric ion as an oxidant or the exclusion of air by a ferric hydroxide coating. Either mechanism is compatible with the experimental data. Thus, methods to keep aqueous ferric ion minimized, e.g., through insolubilization or maintaining high Fe/sup + +//Fe/sup + + +/ ratios, appear to be important in the chemical inhibition of AMD formation.