Sawdust and maize straw were treated by hydrothermal reactions using oxygen or no oxygen from 250C to 374C. The reaction product was separated into aqueous phase, gases, and residue (oil and char). Increased temperatures benefited the conversion of aqueous intermediates and residue to gas. The yields of aqueous products were increased simultaneously under hydrolysis conditions. The increased yield of gaseous phase was ascribed to oxidation decomposition of aqueous and residue. The intra- and intermolecular structures through the hydrogen bonds were partly changed. A plentiful amount of acetic acid was produced in an oxidative environment, but the yield of maleic acid was kept relatively stable.