Tar and char are by-products in the supercritical water gasification of biomass, and they reduce the carbon gasification efficiency of the process. The compound 5-hydroxymethylfurfural (5-HMF), a well-known dehydration product of glucose, is suspected to be a chief intermediate in the polymerization pathway that results in the formation of tar and char. In this study, mixtures of glucose and 5-HMF were used as a feedstock; 5-HMF at concentrations of 0.01 to 0.05 M was added to 1.5 wt% (0.083 M) glucose. The reaction occurred under subcritical conditions (i.e., 350°C and 25 MPa). The gas and char formation rates were observed and compared with those in the case of pure glucose. The experimental results show no significant change in the gas and char yields, even though the 5-HMF concentration was increased by a factor of up to 5; this is an unexpected result. An n-th order kinetic model was developed to describe the gasification and polymerization pathways of glucose under subcritical conditions. The main intermediates for gas and char formation remain as TOC (a lumped liquid product).