In this work, high-performance liquid chromatography (HPLC-UV/RID) is applied to the simultaneous determination of acetic acid, formic acid, acetol, glyoxal, glycolaldehyde and levoglucosan in a by-product in an aqueous liquid phase that is produced by the Hydrothermal Carbonization (HTC) process and in an aqueous bio-oil phase, which comes from a fast pyrolysis process. Both processes were run in forest biomass. For the development and optimization of the proposed method, some chromatographic columns were evaluated based on separation principles of reversed phase and ionic exclusion, although it was previously performed with a solid phase extraction (SPE) process. Concentrations of acetic and formic acids in the liquids of the HTC process ranged from 0.26 to 1.5 % and from 0.14 to 2.7 %, respectively. Concentrations of acetic and formic acids, levoglucosan and glycolaldehyde in the aqueous bio-oil phases ranged from 0.4 – 4.6 %, 0.4 – 1.4 %, 0.13 – 2.5 % and 0.5 – 3.5%, respectively.