The article examines the peat samples found in Tomsk Oblast. The peat samples were divided into two groups. The first group includes the samples of terrestrial, transition, and fen peat. The peat samples of the second group were thermally treated in order to alter their composition and properties. All peat samples were investigated by elemental, functional, and group analysis. The obtained data reveal the changes in such characteristics of samples as type and decomposition degree. According to the comparative analysis of composition and properties of peat samples, it was revealed that preliminary heating of peat in its decomposition gases increases the content of carbon and decreases the content of oxygen and hydrogen in the organic part of the initial matter. As a result, it alters the intensity of stretching bands of oxygen-containing and CH-functional groups. This fact indicates the growth in chemical and biological activity of thermally treated peat samples and their constituents when used in medicine and agriculture. It is revealed that low-temperature treatment of peat samples contributes to increasing the yield of certain peat group components. It is shown that peat treatment is accompanied not only by decomposition, but also it is a unique environment for the reaction between decomposing gases and peat-forming elements, which results in alteration of peat composition. There is high possibility of simultaneous reactions of decomposition and synthesis of new group elements at different speed. These data were applied in estimating the impact of peat heat treatment on its composition alteration. The most intensive changes in group composition are registered in the samples of poorly-decomposed terrestrial peat of a moss group. The conducted research could be a basis for peat enrichment with valuable constituents – humic acids and bitumen.