AbstractThis study aimed to determine glucose and xylose content by acid hydrolysis of wood samples, both unmodified and thermally modified (modification time was 2, 6, 10 hours), using high performance liquid chromatography. Optimization of the hydrolysis process on the native ash wood samples showed that 3 h was the best time in the hydrolysis process. After that time, 58.8% of glucose and 20.8% of xylose were obtained. In turn, chromatographic analysis showed incomplete hydrolysis of ash wood samples, which were modified in a nitrogen atmosphere, especially at shorter times (2 and 6 h) of modification. With longer modification times (10 h), the hydrolysis of ash wood samples was completed. The above mentioned problem was caused mainly by the increase of cellulose crystallinity degree. The decrease of this parameter was observed only after 10 h of thermal modification, which would facilitate the process of acidic hydrolysis. Additionally, it was observed that the thermal modification of ash wood at 190 °C in a nitrogen atmosphere for 10 h caused a drastic decrease in the xylose content (from 20.8% to 8.0%) and only a slight decrease in the glucose content (from 58.8% to 54.9%).