AbstractThe flexural properties, mass losses, and moisture behavior of thermally modified Douglas-fir pretreated with boron or glycerol were examined. Pretreatments were associated with slight, but not significant, reductions in modulus of rupture (MOR) and modulus of elasticity (MOE) of Douglas-fir at different thermal treatment levels. Boron pretreatment had the greatest effect on MOR. MOR of non-pretreated and boron-treated samples increased slightly at the initial stage of thermal treatment and then decreased with rising temperature and time. The MOR of glycerol-treated samples decreased with increasing temperature and time. The thermal treatments employed had no significant effect on MOE. Both temperature and pretreatments improved anti-swell efficiency. Further studies are underway to characterize the nature of the chemical changes associated with the thermal modification process.