The resistance of heat treated (HT) wood to brown rot fungi has been investigated, while the role of the Fenton reaction (FR) in the initial phase of degradation was in focus. Micro-veneers made of Scots pine, were HT with various intensities and their mass losses (ML_HT) were determined before soaking with a solution of Fenton’s reagent containing Fe ions and hydrogen peroxide. The mass loss of the veneers treated that way (ML_FT), their tensile strength loss (TSL_FT) and the H₂O₂ decomposition were observed. The ML_FT, TSL_FT, and H₂O₂ loss decreased with increasing ML_HT of the veneers. Soaking of the veneers in acetate buffer containing only Fe without H₂O₂ revealed that the heat treatment (HT) strongly reduces the Fe uptake by the cell walls. FTIR spectroscopy indicated oxidation of the unmodified control veneers but did not reveal predominant decay of cell wall components; the HT veneers were not changed at all due to FR. It was concluded that the reason for the enhanced resistance of HT wood to FR is attributable to hindered diffusion of Fe ions into the wood cell wall.