In this study, investigation of the thermal properties of Oriental beech (Fagus orientalis) sawdust treated with 0.25, 1.00, and 4.70% aqueous solutions of Adolit KD-5, Wolmanit CX- 8 and Tanalit-E were performed by using thermogravimetric analysis, differential-thermal analysis, and differential-thermal analysis under argon atmosphere. Results were compared with the untreated wood (control). It was found that the treatment with Adolit KD-5, Wolmanit CX- 8 and Tanalit-E decreased the Tmax (maximum degradation temperature) and increased residual char amount with respect to the control sample. Increases in the concentration of applied preservatives promote the char formation. It was found that the char content after pyrolysis experiment had good agreement with the boric acid amount in wood preservatives.