C NMR to determine which type of loose chemical compounds were produced by the thermal treatment. The formation of some toxic polynuclear aromatic hydrocarbons derivatives of phenantrene as well as other classes of polyaromatics compounds was detected. It is most likely that the presence of all such compounds contributes, perhaps to a relatively substantial extent, to the reported resistance of heat treated timber to fungal and other biological attack. Other allegedly non-toxic compounds due to wood constituents degradation were also found, mainly the by-products of lignin pyrolysis. The extent of toxic and non-toxic compounds in the heat treated wood were not quantified, but it can be stated that their proportion appears to be quite small. The lack of quantification of the concentration of these products does not allow to ascertain if the final product is toxic or not, and to what extent. Nonetheless, the amount of polynuclear aromatic hydrocarbon-type derivatives should be quantified and related to the type of heat treatment process undertaken before assigning to this type of wood treatment a completely clean health safety label.