Ulva (Ulva armoricana and Ulva rotundata) are macro algae which are responsible of environmental damages and are a cause of an eutrophication of the water in the French coast (Mediterranean sea, Brittany). In this study, Ulva were gasified in supercritical water in a batch reactor and in Hydrothermal Diamond Anvil Cell apparatus. Experimental conditions evolve between 400 and 550 degrees C at around 250 bar. Results show that a short time (7 min) in supercritical condition is sufficient to obtain a significant conversion rate. H-2 and CH4 concentrations exceed 15 mol.% at high temperature (550 degrees C) and high algae concentrations are not favorable for H-2 production. The solid residue represents between 7 and 20 wt.% of the initial dry material and is mainly constituted of carbon (20 wt.%) and inorganic compounds: salts (KCl, NaCl), CaSO4, SiO2 and CaCO3. The very low dissolved organic carbon (DOC) values of the liquid residue (<0.4wt.%) confirm the high gasification rates. Thiophenes, pyridines, and pyrazines were detected in the liquid phase. Hydrothermal Diamond Anvil Cell (HDAC) experiments were performed to observe gasification stages at the supercritical state. (C) 2015 Elsevier B.V. All rights reserved.