Simulated weathering experiments on coals and shales demonstrate that the critical factors responsible for the generation of acid mine drainage (AMD) are the amounts of total sulfur, total carbonate, and the surface area of the pyrite. Total sulfur and carbonate carbon contents differ markedly among paleoenvironments whose distribution has been mapped for the Alleghenian strata of western Pennsylvania. Freshwater ( Estheria- bearing) shales have a mean total sulfur content of 0.15 percent and a mean carbonate carbon content of 0.54 percent. Brackish ( Lingula- bearing) shales have a mean total sulfur content of 2.40 percent and a mean carbonate carbon content of 0.14 percent. Marine ( Chonetes- bearing) shales have a mean total sulfur content of 0.95 percent and a mean carbonate carbon content of 0.63 percent In the simulated weathering experiments, the amount of acidity, sulfate, and total iron exhibit a well-defined positive linear relation with total sulfur in samples whose carbonate carbon content is ≦0.01 percent. Where carbonate carbon contents are >0.01 percent, the amount of acidity, sulfate, and total iron is considerably less, and the linear relation no longer exists. Anomalously high amounts of acidity, sulfate, and total iron were encountered in both samples devoid of and containing carbonate and were associated with samples containing a high relative percentage of framboidal pyrite and/or pyrite having a high specific surface area. Because determination of the percentage of framboidal pyrite is subjective, direct measurement of pyrite surface area is preferred.