Supercritical Water Gasification (SCWG) is an emerging technology with a great potential to recycle biomass and/or biomass residues and to produce hydrogen-rich gas. SCWG of Olive Oil Mill Waste (OMW) in a continuous reactor at highs waste concentration (7.1-23.5 gO(2)/l) and temperature (550-700 degrees C) had never been studied before. With a residence time ranged between 14.6-52.8 s, the hydrogen yields as well as the removal of Total Organic Carbon (TOC) and Chemical Oxygen Demand (COD) were quantified. As expected from the literature and thermodynamic analysis, the most favourable experimental conditions for H-2 production were the maximum temperature used, 700 degrees C, the longest residence time tested, 40.8 s, and the lowest initial COD, 7.8 +/- 0.1 gO(2)/l, obtaining 112.5 +/- 6.2 mol H-2/kg(OMW) (dry), which is higher than those obtained in from other biomass wastes in similar studies. Finally, different intermediate reaction routes for the processing of OMW by SCWG were suggested.