Fir wood pyrolysis is carried out for extracted, KOH and KCl impregnated, and impregnated-extracted samples. For K+ concentrations in wood of 0.14% (dry mass basis), the effects of KOH are quantitatively higher, but both additives produce a significant diminution in the yields of hydroxyacetaldehyde, furfural, and sugars, the decomposition temperature, and the conversion time. Water extraction of KCl impregnated samples is apt to reproduce again the characteristics of extracted wood pyrolysis. Instead, water extraction of KOH impregnated samples only reports the yields of hydroxyacetaldehyde, acetic acid, and hydroxypropan one to the original values whereas differences in the other variables, especially characteristic times and temperatures and levoglucosan yields, remain very large. Therefore, the action of the impregnated K+ compounds during wood pyrolysis can be attributed to both irreversible modifications in the wood structure during impregnation, whose entity depends on the basicity of the aqueous solution, and intrinsic action of the K+ ion on the activity of the decomposition reactions.