Thermal treatments of two wood species were examined. The temperature load at 200 °C on the wood causes characteristic changes in the chemical composition. The determination of specific alterations was carried out by means of suitable methods, both wet chemical and instrumental analyses. The heat treated wood specimens of beech (Fagus sylvatica) and ash (Fraxinus excelsior) were characterized by means of Carbon analysis. The samples were extracted with organic solvents, cold and hot water. The pH-values were determined in the cold water extracts. Sugar analyses were performed after hydrolysis by means of ion-exchange chromatography. Lignin analyses after thioacidolysis were carried out by means of GC and GC/MS.