The reaction of gaseous KOH with kaolin and mullite powder under suspension-fired conditions was studied by entrained flow reactor (EFR) experiments. A water-based slurry containing kaolin/mullite and KOH was fed into the reactor and the reacted solid samples were analyzed to quantify the K-capture level. The effect of reaction temperature, K-concentration in the flue gas, and, thereby, molar ratio of K/(Al+Si) in reactants, gas residence time, and solid particle size on K-capture reaction was systematically investigated. Corresponding equilibrium calculations were conducted with FactSage 7.0. The experimental results showed that kaolin reached almost full conversion to K-aluminosilicates under suspension-fired conditions at 1100-1450 degrees C for a residence time of 1.2 s and a particle size of D-50 = 5.47 mu m. The amount of potassium captured by kaolin generally followed the equilibrium at temperatures above 1100 degrees C, but lower conversion was observed at 800 and 900 degrees C. Crystalline kaliophilite (KAlSiO4) was formed at higher temperatures (1300 and 1450 degrees C), whereas, amorphous K-aluminosilicate was formed at lower temperatures. Coarse kaolin (D-50 = 13.48 yin) captured KOH less effectively than normal (D50 = 5.47 mu m) and fine (D-50 = 3.51 mu m) kaolin powder at 1100 and 1300 degrees C. The difference was less significant at 900 degrees C. Mullite generated from kaolin captured KOH less effectively than kaolin at temperatures below 1100 degrees C. However, at 1300 and 1450 degrees C, the amount of potassium captured by mullite became comparable to that of kaolin.