In this paper, hydrothermal conversion of biomass is investigated. Part I deals with glucose and part II focuses on woody biomass and pyrolysis oil. Hydrothermal conversion of glucose (250-350 degrees C) has been studied in batch quartz capillary reactors. Kinetics of the overall glucose decomposition was determined and was in agreement with the majority of literature data. Attention was paid to the initial glucose decomposition: primary glucose decay products were identified from literature and used in experiments. It was found that all primary decay components of glucose, with the exception of formaldehyde, produce a kind of char (acetone insoluble product). Characteristic gas (primarily CO2) formation reactions are discussed on the basis of separate tests with primary and other known initial glucose degradation products. Complete mass and elemental balances were obtained for two different temperatures, 300 and 350 degrees C, and various residence times from 10 s to 10 days. It was clearly observed that the product formation as a function of residence time occurs in two distinctly different regimes. The first 5-10 min are characterized by fast changes, whereas after the initial 10 min the changes occur at a much lower rate. It was found that water production, which occurred predominantly in the first 5 min of residence time, was constant (3 mol/mol glucose) and unaffected by temperature or glucose concentration. The yield of the oil product, called here water-acetone soluble (WSS) yield exhibited a maximum at ca. 5 min residence time. After 5 min it was reduced in favor of gas and char, called here water-acetone insoluble (WSIS). However, a certain quantity of WSS is stable even after 10 days residence time. The elemental composition of WSS and WSIS was found to be very similar, which indicates that they are essentially the same product. The highest molecular weight fraction of this product does not dissolve in acetone, whereas the lower molecular weight part does. Compositions of gas, WSS, and WSIS were used as a basis for estimation of the overall reaction enthalpy, which was calculated to be Delta H-r = -1.5 +/- 0.5 MJ/kg. It was found that higher glucose concentrations resulted in more WSIS and less WSS, whereas the gas and water yield did not change. All findings were incorporated into a lumped engineering reaction path and kinetic model of glucose hydrothermal decomposition.