The subcritical and supercritical water gasification of cellulose, starch, and glucose as representative biomass model compounds and biomass in the form of Cassava waste has been investigated in a heated batch reactor. Cellulose and starch are two polysaccharides which have identical chemical compositions based on the monomer glucose but which have different chemical structures and physical properties. The influence of temperature in the subcritical and supercritical regimes of water were examined in relation to the yield of the product gases, oil, char, and water. For the model compounds and the Cassava waste, the main gases produced were carbon dioxide, carbon monoxide, hydrogen, methane, and other hydrocarbons, and there was significant production of oil and char. There were, however, distinct differences between the yields of the different products and the trends in relation to temperature. Cellulose produced a higher yield of char, carbon monoxide, and C-1-C-4 hydrocarbons compared to starch and glucose, but glucose produced the highest hydrogen yield. The Cassava biomass waste produced a char yield similar to that produced by starch, but a lower hydrogen yield.