This study evaluated the effects of boron impregnation and heat treatment on water uptake and tangential swelling of spruce (Picea orientalis), pine (Pinus nigra), beech (Fagus orientalis) and poplar (Populus deltoides) wood species. The samples (10 x 5 x 40 - tangential x radial x longitudinal cm) were impregnated with 4 % boric acid and 4 % borax according to the ASTM D-1413 standard method. After the impregnation, heat treatment was applied on the impregnated test samples in an industrial plant, at four different temperatures and two different durations under steam atmosphere. Soft wood and hard wood samples were subjected to the heat treatment at 212 °C - 220 °C, and at 180 °C - 190 °C, respectively, for 90 and 120 min. Heat treated and impregnated test samples were reduced smaller sizes (3 x 3 x 1.5 cm - tangential x radial x longitudinal) for water uptake and tangential swelling tests. The tests were carried out based on TS 2472. Results indicated that the highest retension was 26.9 kg/m3 in pine wood samples impregnated with BA (26.93 kg/m3). Hardwood retention was lower than soft wood retention. In generally, water uptake and tangential swelling ratios of only heated samples was found lower than non-treated control samples.The combination of heat treatment and BX impregnation was negatively affected water uptake especially in spruce and poplar wood. Borax was more hygroscopic than boric acid. The tangential swelling rates of four wood species impregnated with boric acid and borax were found lower than the non-treated control samples. The tangential swelling rates of spruce and pine wood samples impregnated with boric acid decreased compared to the only heated samples. Heat treatment process significantly affected the remaining boron compounds in wood. Although the steam atmosphere in the oven during the heat treatment, the amount of boron leached from wood was low especially in BX treatment.

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