Introduction. Currently, wood-based construction materials are in high demand in the field of construction around the globe. The study of the original wood composite in order to improve its characteristics through various methods is scientific pre-requisity for such development. When wood burns, it forms a surface coke layer, which can differ in mechanical and thermal stability. These parameters produce a significant effect on the fire resistance rating of building structures due to a change in the reduction rate of the structure useful cross section. Materials and methods. The studies carried out during the work are related to the evaluation of the characteristics of coke layer formed during thermal decomposition of wood. Thermal analysis methods, the scanning electron microscopy method and the water adsorption method were used. The original sapwood of pine, as well as the wood modified with effective fire-retardant agents were used in the course of work. Results. Based on conducted experiments process graphic dependences of thermal decomposition have been obtained, as well as images, obtained by an electronic microscope. In addition, the dependence of adsorptive capacity of wood after thermal decomposition has been obtained. Discussions. As a result of the study, it was found that with the use of modifiers based on organophosphorous compounds, entering into effective reaction with wood, structuring of the surface layer occurs after thermal decomposition while reducing the size of specific surface area of wood. Conclusions. Based on the analysis of experimental data, we can conclude that because of the use of efficient wood surface layer modifiers, thermal decomposition takes place under “mild” conditions that is the shift of the first stage of thermal decomposition to the area of lower temperatures with non-severe exothermic effects. The loss of weight of samples during combustion is significantly reduced. This causes high fire-retardant efficiency and low smoke-generating capacity.