Hydrothermal decomposition of mannose (8 wt.%) in near- and super-critical water was investigated at 500-700 degrees C and 20.0-42.5 MPa with a reaction time of 1 h in the absence and presence of alkali catalyst (K2CO3). Gaseous products, aqueous products, and residue were observed in the batch reactor. The produced gases were carbon dioxide, methane, hydrogen, carbon monoxide, and C-2-C-4 hydrocarbons. The effect of operating parameters (temperature and pressure) on the product distribution was examined in the absence and presence of potassium carbonate. The gaseous product yields were compared with the theoretical equilibrium values that are estimated by Gibbs free energy minimization. In the absence of catalyst, the hydrogen yield was 5.82 mol H-2/mol mannose at 700 degrees C and 20.0 MPa. At this condition, theoretical equilibrium yield of hydrogen was found as 5.78 mol H-2/mol mannose which was very close to the experimental value and addition of K3CO3 increased the hydrogen yield to 10.34 mol H-2/mol mannose. The hydrogen yield increased with increasing temperature and decreasing pressure. Acetic acid was the major component of the aqueous product in gasification of mannose. (C) 2015 Elsevier B.V. All rights reserved.