Thermal modification has been successfully used to improve the durability of Eucalyptus wood from fast-growing forests. Although it is the most common type of modification process, there are some fields for studies on thermally modified wood, particularly about the comparison between processes in closed and open systems. This work aimed to analyse the effect of thermal modification in closed and open systems on the decay resistance of Eucalyptus grandis wood. Short-cycle E. grandis wood, from fast-growing plantations, was thermally modified at target temperatures of 150, 160, and 170 °C in closed system (pressurised), and 170, 190, and 210 °C in open system (atmospheric pressure). A set of untreated wood was separated for comparison. We carried out laboratory tests of i) decay resistance against Trametes versicolor (white rot) and Coniophora puteana (brown rot); ii) hygroscopicity in a climate chamber, resulting in equilibrium moisture content and moisture exclusion efficiency data. Thermal modification in open system at 210 °C resulted in the best treatment to improve the decay resistance of E. grandis wood (durability class 1, DC 1) for both test fungi. After thermal modification at the highest temperature, i.e. 170°C in a closed system, E. grandis was assigned to DC 1-3 against T. versicolor and DC 1 against C. puteana. A conclusive relationship between hygroscopicity and decay resistance was not established. In a future wood chemistry study, we will further address the mode of protective action of thermal modification in open and closed systems.

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