Results from the electron spin-resonance spectroscopy (ESR) analysis indicated that thermally modification of wood can inhibit the generation of phenoxyl radicals during UV irradiation and consequently lead to the increase of the color stability of wood. In order to clarify the evolution and role of extractives in thermally modified wood during the process of weathering, the compositions of acetone extractives from thermally modified Scots pine after exposure in an accelerated weathering tester for different durations were determined by using gas chromatography mass spectrometry (GC-MS). Fatty acids and phenolics were proved to be the main types of extractives in weathered thermally modified Scots pine, also collected were some sugars, terpenes and alcohols. With the progress of weathering, the content of fatty acids decreases significantly while that of phenolics increases. The reduction or even elimination of the fatty acids is presumed to be a physical process, namely, a discharge from wood during exposure to radiation and elevated temperature. The increase of phenolic extractives is mainly due to the photodegradation of other wood components. Phenolic degradation products play the role as “barrier” against further photodegradation of thermally modified wood, among which vanillin or its derivatives appeared to be the most predominant and important ones.

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