The reactions of potassium during combustion of a potassium-ion-exchanged Beulah lignite have been investigated. Combustion was carried out in a drop-tube furnace and CCSEM and XAFS spectroscopy were used to characterize the coal and ash samples. The reactions of the ion-exchanged potassium were quite similar to those of carboxyl-bound sodium in the original coal. The principal K-containing ash phases observed were a K-aluminosilicate glass and a K-Ca sulfate. The K-aluminosilicate glass composition range was clustered around the composition of kalsilite (KAlSiO4). The K-Ca-sulfate mixture exhibited a composition range from K2SO4 to CaSO4, with a mixed (Ca, K-2) SO4 being more abundant than either of the pure sulfate phases.