Fundamental studies of biomass hydrothermal deconstruction reactions have been carried out under realistic conditions using a novel high-pressure differential scanning calorimetry technique. Exotherms related to cellulose and hemicellulose degradation were identified as separate features, with maxima around 280 degrees C and 250 degrees C respectively in the dry state, where curve fitting using a non-isothermal kinetic model was used to postulate the existence of an equivalent lignin exotherm, with a maximum around 240 degrees C, masked by the carbohydrate features. A downward shift in the hemicellulose exotherm was observed on hydration, of around 25 degrees C, which may due to the promotion of preceding hydrolytic depolymerisation reactions, which may reduce the kinetic threshold for subsequent dehydration reactions. No corresponding hydration shift was observed for the cellulose exotherm, consistent with the inaccessibility of the crystalline structure of this biomass component. Differences in hydrated degradation exotherm profiles were observed between wheat-straw, Miscanthus and willow biomass species, which in-part corresponded to differences in enzyme digestibility following hydrothermal treatment. The total willow exotherm exhibited lower enthalpy than straw, of 339 J/g and compared to 510 J/g, with both hemicellulose and cellulose exotherm maxima for willow at higher temperatures, at 233 degrees C and 291 degrees C, compared to 224 degrees C and 281 degrees C for straw, which was consistent with the greater intractability of this woody biomass. The results from the study will be valuable in defining process temperatures and hydration conditions for optimal biomass conversion for downstream thermal and biochemical processing, and also helping to understand phenotypical differences in plant species leading to differing conversion efficiencies.