The aim;of this paper is,to study the alkali volatilization of various biomasses during steam gasification both through thermodynamic calculations and gasification experiments in a fluidized bed. Thermodynamic calculations with a database of simple,compounds predicted two different alkali volatilization behaviors under the typical temperature range of fluidized bed, ?, gasifier (750-100 degrees C). For silica-rich biomasses, calculations predicted the gas phases to be constituted of KCl and HCl with a low K release fraction in gas. The volatilized gas was predicted to condense in the cooled part of the reactor mainly as KCl in agreement with experimental results. For silica poor biomasses, the calculated gas phase was found to be constituted of KCl and 1 KOH with a higher K release fraction in gas than for silica rich biomasses. The volatilized gas was predicted to condense in the, cooled part of the reactor mainly as K2CO3, Which disagreed with experimental results. These discrepancies in gas phase speciation and condensation may be due to the absence of-liquid solution in database used in thermodynamic calculations. and to kinetic limitations in the gas and/or condensed phases.