Catalyst stability and tolerance towards dissolved inorganics are the main challenges for successful hydrothermal gasification of wet biomass. A continuously operating catalyst test rig was built. Synthetic liquefied wood (phenol, anisole, ethanol, formic and acetic acid) was chosen to represent real biomass. After initial screening in a batch reactor, the most promising skeletal nickel catalysts and Ru/C-granular were tested in the new rig, under demanding conditions (high feed concentrations, 10-20 wt%, and high space velocities, 2-34 g(organis) (g(catalyst) h)(-1) at 30 MPa and around 400 degrees C. Skeletal nickel catalysts sintered rapidly but Ru/C was stable during 220 It of testing. The inorganic salt tolerance was examined by co-feeding Na2SO4. Ru/C deactivated over time; a possible mechanism was identified as irreversible sulfate bonding to Ru-III being formed in the redox cycle of biomass gasification. (C) 2007 Elsevier B.V. All rights reserved.