Char-water-slurries from the hydrothermal carbonization (HTC) of paper and brewer's spent grains at 200 C or 240 C were subjected to wet oxidation (WO) by gaseous oxygen at 170 degrees C or 200 degrees C without pre-separation into solid and water phases. The aim was to improve the heat efficiency of the HTC process while preferentially oxidizing the undesired dissolved organic matter (DOM) in the process waters over the chars. Success of this treatment was evaluated on the basis of the distribution of the feedstock chemical oxygen demand (COD) between chars and DOM. Results provide evidence that DOM and char underwent oxidation with a decrease in COD-to-carbon ratios. This fact could be attributed to changes in the concentrations of lactic, acetic, and formic acid quantified by ion chromatography. In oxidized process waters formic and acetic adds clearly dominate. Processes underlying the oxidation of chars are more diverse as revealed by FTIR spectroscopy and affect both cellulose-like and aromatic chars. A promising finding is that DOM was oxidized to a higher degree than the chart and that sufficient heat is released under certain WO conditions to thermally sustain the HTC process. Combined HTC/WO reaction regimes can be optimized for each specific biomass. (C) 2014 Elsevier B.V. All rights reserved.