AbstractCarboxymethyl cellulose salt as a model compound was treated hydrothermally to understand the degradation and formation of several chemicals (furfural, 5‐(hydroxymethyl)‐furancarboxaldehyde, and benzene derivatives) that might be interesting as platform chemicals or fuels. To realize a manageable control, closed quartz capillaries were chosen as reactor. Experiments were conducted at temperatures from 250 (subcritical water) to 450 °C (supercritical water) and reaction times from 1 to 60 min. By studying the gas chromatography‐mass spectrometry analysis results of the liquid products, their formation and degradation were detected. It was observed that certain substances only appear for several minutes and are then degraded, while for others the concentration increases at lower rate and keeps stable even for long reaction times. The formation of several benzoic structures from a non‐ring‐structured precursor showed that different reaction mechanisms are involved. Different reaction temperatures showed good correlation to the formation rate of several substances.