Packed-bed pyrolysis is carried out of beech wood preliminarily impregnated with potassium compounds (KOH, KCl, KC2H3O2, and K2CO3) for ion percentages corresponding to 0-1.8 wt %. The study is mainly aimed at examining the influence of the chemical state of potassium on the global thermicity of packed-bed conversion never examined before. The lowering of primary decomposition temperatures and the enhancement of secondary decomposition of tarry vapors is associated with a remarkable increase in the global exothermicity of the process which highly modifies the conversion conditions and product distribution. Maximum temperature overshoots over a large part of the bed, with respect to the programmed value of 605 K, reach 100-150 K and correspond to minima in the char yields. The quantitative effects are dependent on the content and chemical state of potassium which exerts its action via first the alkaline pretreatment and then catalysis. Moreover, thermogravimetric curves show that the diminution in the primary decomposition temperature is directly related to the K content and the basicity of the additive.