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 °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.