Thermal modification beneficially alters several technological parameters of wood. The changes in the physical parameters are due to the significant alterations of the structure and the chemical composition of wood, which take place during the modification process. These changes are complex and some aspects are still far from being completely understood. Various industrially important hardwoods and softwoods have been treated in an autoclave in N2 atmosphere. The physical (density, L-value, moisture content, bending strength, MOE) and chemical (pH, hemicellulose-, total phenol- and soluble carbohydrate content) parameters have been measured and evaluated. By establishing linear correlations between physical and chemical parameters the chemical changes affecting the physical properties could be investigated and tracked. Very good correlations have been found for hardwood samples, whereas for softwoods only poor correlations have been established. Results could contribute to a better understanding of the reactions of thermal modification, and could furthermore provide a basis for wood species dependent technology optimization in the future.