Studies on the thermal decomposition and on the elemental composition of commercial raw plant materials used in medicine were performed. The 44 independent samples of leaves and flowers originating from different medicinal plant species collected by Medicinal Plants Works "Herbapol" at various factories in Poland were analyzed. The thermal decomposition was performed using the derivatograph. The content of non-metallic (N, P, S, Cl, I and B) and metallic (Ca, Mg, Fe, Mn, Cu and Zn) elements was determined by spectrophotometric techniques after previous mineralization of samples. In order to obtain more clear classification of the analyzed plant materials, principal component analysis was applied. Interpretation of PCA results for three databases (thermoanalytical, non-metals and metals data sets) allows to state, that samples of leaves and flowers from the same plant species in majority of cases are characterized by similar elemental composition and similar course of their thermal decomposition. In this way, the differences in general chemical composition of medicinal plants raw materials can be determined.