Supercritical water gasification (SCWG) is a promising technology for converting wet biomass and waste into renewable energy. While the fundamental mechanism involved in SCWG of biomass is not completely understood, especially hydrogen (H-2) production produced from the interaction among key intermediates. In the present study, formaldehyde mixed with formic acid as model intermediates were tested in a batch reactor at 400 degrees C and 25 MPa for 30 min. The gas and liquid phases were collected and analyzed to determine a possible mechanism for H-2 production. Results clearly showed that both gasification efficiency (GE) and hydrogen efficiency (HE) increased with addition of formic acid, and the maximum H-2 yield reached 17.92 mol/kg with a relative formic acid content of 66.67% in the mixtures. The total organic carbon removal rate and formaldehyde conversion rate also increased to 67.33% and 89.81% respectively. The reaction pathways for H-2 formation form mixtures was proposed and evaluated, formic acid promoted self-decomposition of formaldehyde to generate H-2, and induced a radical reaction of generated methanol to produce more H-2. (C) 2018 Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC.