In this work, the hydrothermal reactions of 'intermediate' compounds of hydrothermal biomass decomposition have been investigated in relation to reaction temperature and time. Experiments were carried out in a batch Hastelloy-C reactor from 250 °C and 4 MPa up to 500 °C and 43 MPa. The results indicate that sodium formate decomposes to mainly H2 (up to 92% yield) with little yield of CO2. The reaction of sodium formate was complete at just above 400 °C, or after 60 min at 350 °C. On the other hand, sodium acetate was stable until around 380 °C, decomposing gradually to produce mainly CH4 (up to 75% yield) and some CO 2. Increasing the reaction temperature beyond 400 °C, and increasing reaction time at 450 °C, led to the increasing production of both CH4 and H2 gases from sodium acetate. These results suggested two possible decomposition patterns for sodium acetate. The predominant mechanism is the reaction with water to produce CH4 and another possible oxidation mechanism that may involve the formation of sodium oxalate at a much higher temperature. The latter mechanism may explain the formation of H2 gas from the hydrothermal reaction of sodium acetate. Selected hydrothermal reactions of sodium oxalate were carried out between 350 and 500 °C. The results showed that at 450 and 500 °C, sodium oxalate yields H2 gas and CO2 almost exclusively. This suggests the possibility of H2 gas production from sodium oxalate as an intermediate product of the reaction of sodium acetate at high temperatures. Generally, this work suggests that sodium formate and sodium acetate may be the important intermediate species that lead to the production of H2 and CH4 gases during the sodium hydroxide-promoted hydrothermal gasification of biomass.