Previous research has not conclusively demonstrated whether KHCO3 formation occurs through the direct reaction of K2CO3 with gas phase CO2 and H2O or via the intermediate formation of K2CO3 (1.5H(2)O). In this study, four sets of experiments, using regenerated and prehydrated K2CO3, were performed in different flue gas component mixtures to fully elucidate the reaction mechanism involved. The extents of carbonation and hydration were quantified by separately tracking CO2 and/or H2O uptakes and releases. Test results revealed regenerated K2CO3 to be most active for KHCO3 formation, while the carbonation of prehydrated samples was limited by their drying rates, proving that K2CO3. (1.5 H2O) cannot be directly converted to KHCO3. Therefore, K2CO3 carbonation and hydration are competing reversible reactions which occur in parallel through direct reaction with the flue gas components. The knowledge gained about the reaction mechanism provides the fundamental direction for future process design to make this technology practical.