The reaction of kaolin powder with K2CO3, KCl, and K2SO4 under suspension-fired conditions was studied by entrained flow reactor experiments and equilibrium calculations. The influence of reaction temperature, K-concentration in the flue gas, molar ratio of K/(Al + Si) in the reactants, and gas residence time on the reaction was investigated. The results showed that the K-capture level (CK) (g potassium reacted by per g kaolin available) of K2CO3 and KCl by kaolin generally followed the equilibrium predictions at temperatures above 1100 °C, when using a kaolin particle size of D50 = 5.47 μm and a residence time of 1.2 s. This revealed that a nearly full conversion was obtained without kinetic or transport limitations at the conditions applied. At 800 and 900°C, the measured conversions were lower than the equilibrium predictions, indicating that the reactions were either kinetically or diffusion controlled. The measured CK of K2SO4 by kaolin was much lower than the equilibrium predictions. Kaliophilite (KAlSiO4) product was predicted by the equilibrium calculations of the K2SO4 capture reaction; however the XRD results revealed that leucite (KAlSi2O6) was formed. Compared with the CK of KOH reacting with kaolin, the CK of K2CO3 was similar, while the CK values of KCl and K2SO4 were both lower.