The objective of this study was to investigate the environmental effects of slaking of surface mine spoils. Both field and laboratory programs were undertaken supplemented by a literature search. The field program consisted of drilling the highwall as well as test pitting in recent 2-, 5-, and 10-year-old spoil piles at four active mining sites in the eastern bituminous coal fields. Pertinent observations were made and samples collected for laboratory analysis consisting of several standard geotechnical, agronomic, and geochemical tests. Durability tests included jar slake, cyclic wet-dry, rate of slake, and slake durability tests. Correlation between field and laboratory portions was made and the results presented. The report concludes with a discussion of the slaking process and associated environmental impacts, proposal of a preliminary classification system for use in premine planning, and presentation of management techniques to optimize the slaking process.