In this work, we examine the evolution of functional groups (carbonyl, carboxyl, phenol, and hydroxyl) during hydrotreatment at 100-200 degrees C of two typical wood derived pyrolysis oils from BTG and Amaron in a batch reactor over Ru/C catalyst for reaction time of 4 h. An aqueous and an oily phase were obtained. The contents of the functional groups in both phases were analyzed by GC/MS, P-31 NMR, H-1 NMR, CHN, KF titration, UV fluorescence, carbonyl groups by Faix and phenols by Folin-Ciocalteu method. The consumption of hydrogen was between 0.007 and 0.016 g/(g of oil), and 0.001-0.020 g of CH4/(g of oil), 0.005-0.016 g of CO2/(g of oil), and 0.03-0.10 g of H2O/(g of oil) were formed. The contents of carbonyl, hydroxyl, and carboxyl groups in the volatile GC-MS detectable fraction decreased (80, 65, and similar to 70%, respectively), while their behavior in the total oil and hence in the nonvolatile fraction was more complex. The carbonyl groups initially decreased having a minimum at similar to 125-150 degrees C and then increased, while the hydroxyl groups had a reversed trend. This might be explained by the initial hydrogenation of the carbonyl groups to form hydroxyls, followed by continued dehydration reactions at higher temperatures that may have increased their content. The P-31 NMR analysis was on the limit of its sensitivity for the carboxylic groups to precisely detect changes in the upgraded nonvolatile fraction; however, the more precise titration method showed that the concentration of carboxylic groups in the nonvolatile fraction remains constant with increased hydrotreatment temperature. The UV fluorescence results show that repolymerization increases with temperature, starting as low as 125 C. ATR-FTIR method coupled with deconvolution of the region between 1490 and 1850 cm(-1) was shown to be a good tool for following the changes in carbonyl groups and phenols of the stabilized pyrolysis oils., The deconvolution of the IR bands around 1050 and 1260 cm(-1) correlated very well with the changes in the P-31 NMR silent 0 groups (likely ethers). Most of the H2O formation could be explained from the significant reduction of these silent 0 groups (from 12% in the fresh oils, to 6 to 2% in the stabilized oils) most probably belonging to ethers.