We gasified cellulose in supercritical water, in the absence of heterogeneous catalytic effects, by using quartz reactors. We also report the first systematic study of the effects of temperature, cellulose loading, water density, and reaction time on the production of H-2, CH4, CO, and CO2 from supercritical water gasification. The results show that the total gas yields and H-2 mole fraction are lower in quartz reactors than in stainless steel reactors, suggesting that the gases from previous studies in metal reactors arise from both homogeneous and heterogeneous reactions, even in the absence of an added catalyst. The rate of formation for all gas species increases with temperature. Manipulating cellulose loading and water density provides an efficient means to control the product selectivity, since the relative amounts of H-2 and CH4 were strongly influenced by these two process variables.