Historically, fly ash generated in the United States was dry disposed on land or sluiced into ponds for solid/liquid separation, while freshly generated ash was sold for application in concrete. Recent seasonal shortages in the fly ash market have generated interest in beneficial application of the ponded ash as a supplementary cementitious material (SCM); however, exposure to water during disposal and permanent storage has impacted the morphology and geochemistry of the ponded coal fly ash. This study examines the mineral phases and carbon content of 27 samples of weathered fly ash, to determine how weathering changed the ash. Thermogravimetry analysis indicates that weathered fly ashes have hydrated phases that contribute to LOI values over 6% (the maximum limit for ash use as SCMs in concrete). Because these ashes have been exposed to water, a significant percentage of total mass loss is due to mineral volatilization and dehydration during thermal treatment. The general inorganic composition of weathered fly ashes is comparable to unweathered fly ashes, and all weathered ashes met the oxide and fineness requirements of Class F fly ashes. The differences between weathered and unweathered fly ashes are primarily related to differences in morphology and volatile behavior during thermal treatment.