Hydrothermal gasification is considered to be a promising technology for the efficient conversion of wet biomass residues into fuel gas. Unlike conventional gasification methods, such as methane fermentation or thermal gasification, Supercritical Water Gasification (SCWG) produce a gas fuel, it does not require a previous drying process and it has many advantages from the energy, environmental and financial point of view. This work is particularly valuable since it studies the use of real biomass residues; i.e. Olive Mill Wastewater (OMW) treated with both catalytic and non-catalytic SCWG to produce hydrogen. The increasing amount of OMW is becoming a serious environmental problem in olive oil-producing countries. SCWG process of OMW in a batch reactor at 530 degrees C and 250 bar was studied, and the effect of the catalysts on the process was evaluated in terms of biomass conversion (COD removal), gases yields (H-2, CH4, CO2 and CO) and H-2 selectivity.