Thermal modification of wood causes several chemical changes in wood cell walls, leading to improvement of dimensional stability and durability against fungal attack, and a decrease of some mechanical properties. Scots pine (Pinus sylvestris) samples were thermally modified in nitrogen atmosphere at 220 °C, 240 °C and 250 °C. After modification, the samples were weighed to determine mass loss. A small block test was used to test durability against white rot fungi (Poria placenta). Using GC-MS, the chemical changes occurring during thermal modification were observed. C, H and O content variation was analyzed, and O/C ratio calculated. The results show positive correlation among mass loss, temperature and time of heat treatment. Wood durability is in positive correlation with the degree of modification. The samples with 15 % mass loss were completely resistant against the fungus, irrespectively to temperature. The reason for better durability is degradation of main wood components, mainly hemicellulose, the main nutrient for fungal growth. Thermal degradation decreases O/C ratio. This value is in linear relationship with the degree of modification, allowing the determination of mass loss indirectly.