The purpose of this study was to examine the hydrothermal (HT) treatment of three invasive aquatic plants (i.e., Lemna sp., Hydrilla sp., and Eichhornia sp.) with respect to the generation of semi-volatile hydrocarbon product mixtures and biomass volume reduction. Identical HT treatments yielded similar semi-volatile product mixtures for Hydrilla sp. and Eichhornia sp. versus a significantly different mixture for Lemna sp. Pre-treatment (i.e., control) extracts of the plant substrates showed no semi-volatile hydrocarbons. Post-HT treatment product mixtures were comprised of complex mixtures of compounds including branched and unbranched alkanes and alkenes as well as light aromatics including substituted benzenes and phenols. All three plant HT product mixtures were dominated by phenol, C, alkyl phenols, and oxygenated cycloalkenes. Lemna sp. products showed much more diverse distributions of C-2-C-5 alkyl benzenes, alkyl indanes, and alkyl naphthalenes at higher relative levels. Other products from the Lemna sp. HT treatment included C-2-C-4 phenols, and alkyl indole and indanol compounds. Results of wet chemical analyses showed that a major difference between Lemna sp. and the other two plants was significantly higher extractives levels in the former. It was found that this fraction accounted for much of the complexity in HT product mixture of the Lemna sp. biomass. For all HT treatments the substrate mass was reduced by 95% or more. (C) 2007 Published by Elsevier Ltd.