The degradation of biomass is studied in the ranges of 330-410 degreesC and 30-50 MPa and at 15 min of reaction time. To characterize the chemistry of biomass degradation, key compounds which are intermediates of the biomass degradation are identified and quantified. These key compounds are used to compare the results from earlier studies of model compounds such as, e.g., glucose or cellulose, with biomass degradation in order to identify chemical reaction pathways. Key compounds identified are phenols (phenol and cresols), furfurals, acids (acetic acid, formic acid, lactic acids, and levulinic acid), and aldehydes (acetic aldehyde and formic aldehyde). In addition, sum parameters such as the total organic carbon content and the composition of the formed gas phase are used to describe the biomass degradation. The results are compared with the hydrothermal upgrading process and with the well-known gas-phase gasification processes. The influence of the change of water properties from subcritical to supercritical conditions on the biomass degradation is also discussed. These comparisons show that most of the main reaction pathways detected by the key compounds can be understood by the studies of model compounds. On the other hand, biomass is much more complex because biomass contains a lot of different substances. Especially, the influence of salts is significant and, in addition, rather complex.