Thermal modification is a process which is used to improve the quality of wood (reduced hygroscopicity, improved dimension stability and increased resistance to wood pests). Ash wood was selected for the process because of its good mechanical properties and high density. The wood was modified in the absence of oxygen at 180, 190, 200, 210 and 220 °C. Thermally modified samples were then analysed by thermogravimetry in the temperature range from 25 to 600 °C under argon flow. Calibration curves were constructed showing the mass loss during the TG measurement as a function of the mass loss during thermal modification. All calibration curves show a linear dependence, with the highest correlation coefficient in the range from 130 to 280 °C, 130 to 290 °C, and 130 to 300 °C. The adsorbed moisture of thermally modified samples was checked by a humidification process in a controlled atmosphere with known humidity. Samples were moistened in a closed desiccator that contained saturated solutions of either Mg(NO3)2 or KNO3. From the data of the mass loss during drying of the previously moistened samples and the mass loss during thermal modification, calibration curves were again plotted. Both calibration curves showed exponential dependence, while the calibration curve from the humidification processes with Mg(NO3)2 had a higher correlation coefficient.