A comparative study of the modification of chemical composition of softwood and hardwood by torrefaction at different temperatures (150-300 degrees C) was carried out using microwave treatment and convective heating. The nonthermal effect was established for softwood, revealing increased aromaticity of the samples obtained under microwave treatment. A difference of the effects of microwave treatment on softwood and hardwood was revealed. In the case of softwood, both carbonization processes and unsaturation development of the matrix structure were promoted, whereas for hardwood only promotion of the carbonization processes was observed. Formation of extractable compounds as the result of wood microwave treatment was monitored, and the significant increase in formation of hydrophilic extractable compounds in the range of 210-280 degrees C (hardwood) and 250-300 degrees C (softwood) was found. The destructive changes of lignin macromolecules at relatively low temperatures (beginning with 150 degrees C) were confirmed by decreasing Klason lignin content and simultaneous significant increase in the yield of acid-soluble lignin from both wood species. The structure of the matrix formed in the result of softwood microwave treatment at 230-280 degrees C was much more thermostable than both the original wood and the solid material obtained by convective heating at the respective temperatures.