The hydrothermal gasification is a promising process for the conversion of wet biomass. Coke formation can occur during the heating up, leading to a decrease in the gasification efficiency or even to a blockage of the plant. A new work-up procedure was developed for separating and measuring the amounts of both tars and coke, combined with a compound-specific HPLC analysis of the aqueous product phase. The glycerol experiments were conducted in a temperature range of 300-430 degrees C, residence times of 5-120 min and feed concentrations up to 30 wt.% at pressures around 30 MPa. Additional experiments were performed with glucose, phenol, hydroquinone, and ethylene glycol. Coke formation was observed at subcritical temperatures and long residence times. The highest rate of coke formation occurred in the temperature range of 350-370 degrees C. Phenol and hydroquinone can be ruled out as precursors for the formation of coke. (C) 2012 Elsevier B.V. All rights reserved.