In this study, the hydrothermal gasification of biomass in supercritical water is investigated. The work is of peculiar value since a real biomass, olive mill wastewater (OMW), is used instead of model biomass compounds. OMW is a by-product obtained during olive oil production, which has a complex nature characterized by a high content of organic compounds and polyphenols. The high content of organics makes OMW a desirable biomass candidate as an energy source. The hydrothermal gasification experiments for OMW were conducted with five different reaction temperatures (400, 450, 500, 550 and 600 C) and five different reaction times (30, 60, 90, 120 and 150s), under a pressure of 25 MPa. The gaseous products are mainly composed of hydrogen, carbon dioxide, carbon monoxide and C(1)-C(4) hydrocarbons, such as methane, ethane, propane and propylene. Maximum amount of the gas product obtained is 7.71 mL per mL OMW at a reaction temperature of 550 degrees C, with a reaction time of 30 s. The gas product composition is 9.23% for hydrogen, 34.84% for methane, 4.04% for ethane, 0.84% for propane, 0.83% for propylene, 49.34% for carbon dioxide, and 0.88% for minor components such as n-butane, i-butane, 1-butene, i-butene, t-2-butene, 1,3-butadiene and nitrogen at this reaction conditions. (C) 2011 Elsevier B.V. All rights reserved.