AbstractA series of experiments were carried out to investigate photo-degradation of thermally modified (at 210oC and minus 0.9 bars for two hours) and non-modified spruce wood [Picea abies L (Karst)], coated with transparent and semitransparent (with 3% pigment content) acrylic coatings during artificial UV light irradiation for 200 hours. Photo-degradation was evaluated in terms of colour changes throughout the irradiation period at an interval of 50 hours, along with IR and EPR spectroscopic study. One set of modified and non-modified woods was painted with coatings, while the other set was covered with free films made of coatings, just to simulate coated wood. The colour changes for both modified and non-modified wood samples without paint-coat or free film cover were comparable to that of wood samples with paint-coat and free film cover for transparent coat type, which indicated its ineffective-ness to prevent photo-degradation of wood underneath. However, the colour changes for both modified and non-modified wood samples with paint-coat and free film cover were much lower than those of samples without paint-coat or free film cover for semitransparent coat type, which might be due to hindrance of transmission of light energy through pigment to reach the underlying wood surface. On the other hand, whole substrate-coating system showed better photo-stability, when thermally modified wood was used as substrate. However, the colour changes of paint-coated and free-film covered samples for both modified and non modified woods might be due to colour changes of wood specimen underneath, because free films of both the coat types showed negligible colour change during UV irradiation.