Fourier-transform near-infrared spectroscopy (FT-NIR) was used as none-destructive method to determinate changes in the chemical structure of heat-treated wood. For this purpose, pine sapwood (Pinus sylvestris L.) was treated at different temperatures (from 100 °C to 240 °C) and for three durations (1, 3 or 5 hours). The effects of chemical changes on the FT-NIR spectra are linked to absorbance changes of functional groups (-OH,-CH,-CO and-CH2) of lignin, hemicelluloses and cellulose. Gradual degradation of amorphous portion of cellulose was caused by high temperature, while crystalline and semi-crystalline portions of cellulose seem to be less affected by the thermal treatment. The effect of various intensities of heat treatment on chemical changes of wood polymers varied depending on temperature and duration. Presentation of spectra in the form of the xylograms shows clear tendency of degradation kinetic. Evaluation of thermal stability of selected wood component and/or comparison of the influence of modification process parameters can be carried out.