Using a pilot-scale chamber with an interior capacity of 340 L, European aspen (Populus tremula) wood boards were thermally modified (TM) under pressure in nitrogen at a maximum temperature of 160–170 °C, for 60–180 min, and with an initial nitrogen pressure of 4–5 bar. After the TM process, aspen wood was characterised by dimensional changes, mass loss (ML), equilibrium moisture content (EMC), antiswelling efficiency (ASE), cell wall total water capacity (CWTWC), modulus of rupture (MOR), modulus of elasticity (MOE), and Brinell hardness (BH). This work offers fresh insights into the characteristics of aspen wood following a closed TM process in pressurised nitrogen. TM caused ML of 5.4–14.5% and shrinkage in all anatomic directions. The ASE ranged from 22 to 70%, while the CWTWC was reduced from 35% to 11–27%. After treatment, EMC and volumetric swelling (VS) were more than twice as low as in untreated wood. Although MOE values increased and the average MOR reducedUsing a pilot-scale chamber with an interior capacity of 340 L, European aspen (Populustremula) wood boards were thermally modified (TM) under pressure in nitrogen at a maximumtemperature of 160–170 ◦C, for 60–180 min, and with an initial nitrogen pressure of 4–5 bar. After theTM process, aspen wood was characterised by dimensional changes, mass loss (ML), equilibriummoisture content (EMC), antiswelling efficiency (ASE), cell wall total water capacity (CWTWC),modulus of rupture (MOR), modulus of elasticity (MOE), and Brinell hardness (BH). This work offersfresh insights into the characteristics of aspen wood following a closed TM process in pressurisednitrogen. TM caused ML of 5.4–14.5% and shrinkage in all anatomic directions. The ASE rangedfrom 22 to 70%, while the CWTWC was reduced from 35% to 11–27%. After treatment, EMC andvolumetric swelling (VS) were more than twice as low as in untreated wood. Although MOE valuesincreased and the average MOR reduced following TM, the changes were not important. The TMaspen wood tangential surface’s BH dropped and was noticeably lower than the radial surface’s B