Bio-oils rich in monoaromatic phenolic compounds were produced by a hydrothermal treatment in a batch reactor from organosolv lignin derived from beech wood. Reaction temperatures and times were varied (270-350 degrees C and 10-120 min, respectively). Increase in the temperature at a particular reaction time had a positive impact on the bio-oil yields, which varied from 8.0 wt% to 14.6 wt%, based on the original amount of dry lignin. GC-MS analysis of bio-oils revealed that the yields of monoaromatic compounds ranged from 22 to 65 wt% of bio-oil depending on the reaction conditions. Syringol (8.9-22.8 wt% of bio-oil), guaiacol (2.6-9.3 wt % of bio-oil), pyrocatechol (0-12.4 wt% of bio-oil), 3-methoxycatechol (0-21 wt% of bio-oil), 4-methylsyringol (0.5-5.9 wt% of bio-oil), and syringaldehyde (0-9 wt% of bio-oil) were identified as the major aromatic compounds. In addition to bio-oil, gaseous components, water solubles, char, and undegraded lignin were formed in the experiments. The mass balances of the experiments were constructed. The results show that monoaromatics can be produced at a moderate yield through uncatalysed lignin hydrothermolysis; char formation remains as an obstacle, however, and its prevention calls for the usage of catalysts and/or organic solvents.