Inorganic constituents present in UCG process effluents can be strongly correlated with changes in coal and overburden mineralogy. An analysis of the Hanna, Wyoming, UCG site provides insight on how inorganic constituents enter the system. A comparison of preburn versus postburn mineralogy indicates that carbonates, clay minerals, micas, and sulfurous minerals break down structurally because of high temperatures. This decomposition releases calcium, magnesium, silica, and aluminum for rebonding in high-temperature stable silicate and al um1 nosi1icate minerals. It also releases constituents such as iron, sodium, sulfur, and boron into the UCG process effluent. Other constituents such as calcium, copper, magnesium, manganese, potassium, and sodium are predominantly introduced into the groundwater by leaching. Groundwater quality data and analyses of other process effluents support these interpretations. Complete chemical and mineralogical characterization of the coal, coal ash, and immediate overburden is an important tool in determining inorganic constituents that affect groundwater quality at UCG sites. Early planning offers substantial economic incentives by allowing treatment while the constituents are confined to the cavity before migration affects much larger areas.