Biomass model compounds (cellulose and lignin alkali) and their mixtures were gasified at sub- and super-critical water conditions in the absence and presence of alkali catalyst (K2CO3). Hydrothermal gasification was performed in batch reactor at temperature range of 300-600 degrees C and pressure range of 90-410 bar with a reaction time of 1 h. Product yields of gaseous, aqueous and residue were investigated for model compounds (cellulose/lignin alkali) and their mixtures with weight ratios of 80/20, 60/40, 40/60 and 20/80 (wt./wt.). These mixtures were arranged to define the interaction between degradation products of model compounds of lignocellulosic biomass. In addition, gaseous and aqueous product compositions were identified to highlight prevailing reaction pathways during decomposition of model compounds. The highest hydrogen and methane yields were reached at 600 degrees C in the presence of K2CO3. (C) 2016 Elsevier B.V. All rights reserved.