Abstract: After thermal modification at 210 °C using the Thermowood method, changes in Pinus radiata D. Don wood cell wall components and hygroscopicity were studied for subsequent comparison with recently felled wood of the same species in samples from the same region of provenance (Basque Country, Spain). To this end, samples were characterised by sorption isotherms at 15, 35 and 50 °C fitted to the Guggenheim–Anderson–de Boer (GAB) model, the chemical composition was obtained by high performance liquid chromatography (HPLC) and infrared spectroscopy (FTIR), and crystallinity and structural organisation of cellulose (crystal orientation) were determined using powder and 2D X-ray diffraction (XRD). Heat treatment caused the following changes in the wood: a decrease in the equilibrium moisture content (EMC); a smaller hysteresis area and therefore more hygroscopically stable wood; a decrease in the hemicellulose content; an increase in the relative percentage of cellulose, lignin and extractives; and a higher degree of crystallinity and crystal orientation of the cellulose. The reorganisation of cellulose could be explained by epitaxial growth of cellulose starting from the highly oriented crystalline regions during the recrystallisation process. All these chemical and structural changes induced by heating could explain the reduction in hygroscopic properties of wood, as well as its stability.