Studies on the thermal decomposition of commercial raw plant materials used in medicine were performed. 144 independent samples of plant materials — herbs, leaves,flowers, inflorescences, fruits, roots, rhizomes and barks, collected by Medicinal Plant Works‘Herbapol’, were analyzed. Thermal decomposition was performed using OD-103 Derivatograph. As a result of analysis, it was established, that thermal decomposition of majority of samples proceeds through three stages. The analysis of fruits revealed, that their thermal decomposition proceeds in four stages. In order to obtain a more clear classification of the analyzed plant materials principal component analysis (PCA) was applied. Interpretation of the PCA results allows to state, that samples of raw materials from the same plant species in majority of cases are characterized by similar course of thermal decomposition due to similar chemical composition. In this way the differences in general chemical composition of medicinal plants raw materials can be determined.