Co-liquefaction of secondary pulp/paper-mill sludge (solids concentration: 1.6 wt%) and waste newspaper with a total solids concentration of 11.3 wt% was investigated with and without the addition of catalysts in a 75 ml Parr High-Pressure reactor at temperatures of 250-380 degrees C for 20 min. The yield of heavy oil (HO) without catalyst was between 16.7 and 28.0 wt% within this temperature range, and peaked at 350 degrees C. The addition of HCO2H, FeS, or KOH at 5 wt% of the total solids (on a dry basis) was found to enhance the HO yield at 300 degrees C. particularly HCO2H, which increased the yield of HO from 24.9 to 34.4 wt%. More interestingly, synergistic effects between secondary pulp/paper-mill sludge and waste newspaper were observed in the co-liquefaction operations. For example, the HO yield attained was 26.9 wt% at 300 degrees C in the co-liquefaction of the mixture of 33 wt% sludge and 67 wt% waste newspaper, and was noted to be 9 wt% and 6 wt% higher than the yields obtained from liquefaction of sludge and waste newspaper alone, respectively. The HOs from liquefaction or co-liquefaction at 300 degrees C for 20 min exhibited significantly higher energy contents (HHV >= 30 MJ/kg), almost doubled those (16 MJ/kg) of the original feedstocks. (C) 2010 Elsevier Ltd. All rights reserved.