This work examined the transformation of glucose in hydrothermal (HT) reaction systems. Isotopic studies and comparative product analysis were used to gain insight on the most prevalent HT reactions. HT in H2O16 and H2O18 showed that transformation of glucose to gaseous and semi-volatile compound mixtures was dominated by hydration/oxidation reactions by water molecules in the reaction medium. Dissolved molecular oxygen and oxygen from the substrate were minimally involved in the observed oxidation reactions. HT of glucose in D2O indicated that most of the hydrocarbons, phenolics, and related reduced compounds in the products were from reactions involving unsaturated fragments of the substrate molecules. Except for furans and alkyl furans, there was no evidence of reduction and aromatization reactions acting directly on glucose rings: these molecules first underwent fragmentation and desaturation reactions prior to the formation of semi-volatile products. These reactive fragments underwent further reactions to form semi-volatile products such as Diels-Alder additions. Timed-release of NaBH4 to the HT system decreased the relative amount Of CO2 generated and increased the yields of hydrocarbon products. This indicated that levels of reducing hydrogen species limit the yields of reduced products such as hydrocarbons and allow hydration/oxidation reactions to "steal" substrate carbon atoms and generate oxidized products, notably CO2. Co-reactants entering the HT reaction mixture as a result of containerization (e.g., Ni) also influenced the products of HT reaction. The presence of leached reactor metals in the medium was found to affect both quantities and types of compounds found in both the volatile and semi-volatile phases. (C) 2009 Published by Elsevier Ltd.