H-2, CH4, CO and CO2 yields were measured during supercritical water gasification (SCWG) of primary paper waste sludge (PWS) at 450 degrees C. Comparing these yields with calculated thermodynamic equilibrium values offer an improved understanding of conditions required to produce near-equilibrium yields. Experiments were conducted at different catalyst loads (0-1 g/g(PWS)) and different reaction times (15-120 min) in a batch reactor, using either K2CO3 or Ni/Al2O3-SiO2 as catalyst. K2CO3 up to 1 g/g(PWS) increased the H-2 yield significantly to 7.5 mol/kg(PWS). However, these yields and composition were far from equilibrium values, with carbon efficiency (CE) and energy recovery (ER) of only 29% and 20%, respectively. Addition of 0.5-1 g/g(PWS) Ni/Al2O3-SiO2 resulted in high H-2 and CH4 yields (6.8 and 14.8 mol/kg(PWS)), CE of 84-90%, ER of 83% and a gas composition relatively close to the equilibrium values (at hold times of 60-120 min). (C) 2015 Elsevier Ltd. All rights reserved.