The migration of K during biomass thermal conversion can cause severe ash-related problems. K2CO3 and KCl are two important inorganic forms of K that appear during biomass thermal conversion. In this study, a fixed-bed reactor was used to investigate the effect of H2O vapor and SiO2 on the migration characteristics of these two inorganic K species. The results showed that K is released through the decomposition of pure K2CO3 during pyrolysis. K release from K2CO3 at 800 C was not obvious, and the release rate increased significantly with temperature but mainly depended on the decomposition rate of K2CO3. The presence of H2O vapor can promote K release by reacting with K2CO3 and forming volatile KOH. When SiO2 was present, a rapid reaction occurred between SiO2 and K2CO3, and this reaction inhibited the release of K significantly. The reaction rate increased significantly with temperature, and H2O vapor promoted the reaction of SiO2 and K2CO3. In a pyrolysis atmosphere, KCl can be released by direct evaporation and cannot react with SiO2; by contrast, KCl can react with SiO2 to form insoluble K silicates in the presence of steam. With increasing temperature, the release rate of KCl increased faster than the K silicate formation rate, resulting in a decrease in the proportion of insoluble K silicates generated. The results obtained in this work provide a basis for understanding the migration of inorganic K during biomass thermal conversion.