The release of inherent potassium may cause severe problems in biomass thermal utilization. A fast-pyrolysis experiment was carried out with rice straw at 400-1000 degrees C in a fixed-bed reactor. The release of K and Cl to gas and the occurrences of K in residues during devolatilization and char pyrolysis were quantified on the basis of the analysis of rice straw and char samples after devolatilization and pyrolysis for 5 and 30 min. K release after different pyrolysis times all increased with the temperature. The contribution of devolatilization to K release exceeded char pyrolysis above 900 degrees C. K existed as organic and inorganic K in rice straw with a similar amount, and the original inorganic K is mainly KCl. During devolatilization, organic K dominated K release below 800 degrees C, while KCl took the place above 900 degrees C. Significant char K and K2CO3 were generated after devolatilization. During char pyrolysis, KCl dominated K release below 900 degrees C, while K2CO3 took the place at 1000 degrees C. Insoluble K silicates were found to form at 900 and 1000 degrees C, while the char structure restricted the formation amount to about 7%.