5-Hydroxymethylfurfural (5-HMF) is one of the intermediates in char polymerization, a side reaction that occurs in the supercritical water gasification (SCWG) of biomass. In this paper, we report the kinetic investigation of the polymerization of 5-HMF during solid char formation in an acidic environment. This is due to the fact that 5-HMF coexists with other product acids during biomass decomposition. Mixtures of 5-HMF and HCl were treated under subcritical conditions (350 degrees C; 25MPa). The concentration of 5-HMF ranged from 0.01 to 0.05 M, while that of HCl was 0.01 M (at room temperature). The rates of gas and char production from 5-HMF in experiments with and without acid addition were compared. The acid catalyzed the gasification and polymerization pathways of 5-HMF, especially the decomposition of 5-HMF to TOC, the rate constant of which was increased by two orders of magnitude.