Data are presented describing chemical and mineralogical composition of spoil material and chemical quality of groundwater at 2 Wyoming mine sites. Samples were collected at Medicine Bow-Seminoe Number 1 mining area in the Hanna basin and at the Cordero Mine in the Powder River basin. The data collected from these sites, along with similar data from other coal-mining states in the West, are used to evaluate methods used in predicting post-mining groundwater quality. The data include mineral-composition analyses, paste-extract analyses, and sulfur-forms analyses of coal spoil, chemical analyses of water from batch-mixing experiments; and analyses of water samples collected from wells in the coal aquifers and from wells in the saturated spoils. (USGS)