Spruce and fir samples were thermally modified in vacuum at temperatures from 170 to 215 °C. The mass losses caused by heat treatment were evaluated for modified samples. FTIR spectra of the heat treated and control samples were recorded in the transmision technique. In these spectra, the areas of single bands were measured and compared to the reference bands at 2900 and 1510 cm-1 to get the relative areas of single bands. Relative areas were then compared to the mass loss caused by thermal modification, and a linear relationship was observed for bands at 1636, 1605, 1035 and 897 cm-1. TG was also used for analysis of control spruce samples and samples modified at 190 and 210 °C. Mass losses observed by thermal analysis between 100 and 800 °C are linearly proportional with the mass losses caused by a heat treatment. Increasing the mass loss caused by a heat treatment the mass loss detected by the thermal analysis decreases.