The catalytic gasification in supercritical water of the water soluble fraction of bio-oil, either obtained directly by phase-separated pyrolysis-oil from ligno-cellulosic biomass or by hydrotreatment of that oil, is reported in this study. Several heterogeneous metal catalysts Pt, Pd, Ru, Rh, and Ni supported on alumina were tested for their gasification efficiency (GE). The GE for the metals is decreasing in the order Ru > Pt > Rh ∼ Pd > Ni. For optimum H2 selectivity the order of the catalysts is Pd > Ru ∼ Rh > Pt > Ni. Pd catalysts with different supports have been screened and no significant changes in the GE were found for the different supports. However, the composition of the product gas differed significantly with the support type. High H2 selectivity was obtained with Al2O3 and Ce–ZrO2 supports. With increasing the organic concentration from 5 to 50 wt% the GE as well as the H2 and CO2 selectivities dropped significantly. High reaction temperatures, long residence time, low feed stock concentrations and high catalyst loadings favored the high carbon to gas conversion. The aqueous wastes streams obtained from the hydrodeoxygenation process for the pyrolysis oil are easier to reform in supercritical in comparison to the feedstocks obtained directly as pyrolysis condenser fraction or as phase-separated aqueous fraction. Complete conversion of the made-up and the fast pyrolysis condenser fraction was obtained at low feed concentrations (5 wt%) using a continuous flow reactor in the presence of Ru/Ce–ZrO2 catalyst. However, the catalyst quickly deactivated with the made-up fraction but the same catalyst retained its stability and activity with the pyrolysis condenser fraction during the 3 h test run. The supercritical water gasification seems therefore a very suitable step for treating the aqueous phase obtained after hydrotreatment of pyrolysis oil in a (biomass) refinery concept.