Abstract: The catalysis of potassium carbonate on the steam gasification of coals was investigated in a fixed-bed reactor. The interaction of potassium with mineral matter in ten coals was examined. The gasification residue after leaching with water was analysed by X-ray diffraction (XRD) and X-ray fluorescence (XRF). The results show that potassium aluminum silicate is the major crystallite derived from the interaction of potassium with mineral matter. As the ash is in low calcium content, the molar ratio of non-recoverable potassium to aluminum is one. However, as the ash is in high calcium content, the calcium can fix a large number of aluminum in the form of gehlenite (Ca₂Al₂SiO₇), which would inhibit the deactivation of potassium.