This study deals with an approach to classify thermally modified wood. In recent years, this material has gained a strong foothold in interior and exterior use. To guarantee the fulfillment of customer requirements as well as specific aesthetic preferences, quality grades for thermally modified European Ash (Fraxinus Excelsior) and European Beech (Fagus Sylvatica) wood have to be defined. Only consistency in the expected properties can guarantee the long-term success of thermally modified hardwoods on the market. This study shows, while the modified wood can be classified by color, the moisture adsorption of the wood depends significantly on the type of treatment. Moreover, it was concluded that the color information can also be used to define various grades of thermally modified hardwoods according to the bending strength. Guidelines to determine the aesthetic and technical quality of thermally modified wood have to be established, which this study underlines. These findings will be useful to develop international and national standards for thermally modified hardwoods for various applications.