The hydrothermal gasification of Posidonia oceanica was investigated in a batch reactor without adding any catalysts. The experiments were carried out in the temperature range of 300-600 degrees C with different biomass loading ranges of 0.04-0.12 (g/mL) in the reaction time of 1 h, The product gas was composed of hydrogen, methane, carbon dioxide, carbon monoxide and a small amount of C-2-C-4 compounds. The results showed that the formation of gaseous products, gasification efficiency and yield distribution of produced gases were intensively affected by biomass loading and temperature. The yields of hydrogen (1037 mol/kg) and methane (6.34 mol/kg) were attained at 600 degrees C using biomass loading of 0.08 (g/mL). The results are very promising in terms of deployment of the utilization of marine biomass for hydrogen and/or methane production to industrial scale applications, thereby proposing an alternative solution to the landfill of P. oceanica residues. (C) 2015 Elsevier Ltd. All rights reserved.