Hydrothermal catalytic gasification of diluted (0.1 M) biomass (glucose) solution into hydrogen, methane, and carbon dioxide gases in hot, compressed water has previously been achieved. However, efficient gasification was limited to low concentrations, mainly because the glucose-water system was found to react in a homogeneous phase at a relatively moderate temperature (238-250 degrees C). These reactions yielded precipitates that blocked the catalyst surface. In order for this technology to be practical and economically feasible, high glucose concentrations, higher than 1 M, should be processed. The phase behavior of the concentrated glucose solution was studied in supercritical water using a diamond-anvil cell and a continuous-flow reactor. The homogeneous phase reactions were triggered by the presence of acidic media and heat released during the hydrothermal processes. Dehydration of glucose leading to the formation of 5-hydroxymethylfurfural (5-HMF) is suggested as the major step in the evolution mechanism of the homogeneous phase reactions. 5-HMF tends to polymerize forming precipitates of oligomers with a high degree of polymerization. A novel reactor design has been developed to promote selective biomass gasification to hydrogen while preventing undesired precipitation reactions.