The aim of this work was to investigate the principal structural and physico-chemical changes of bio-oils associated with liquefaction of swine manure with crude glycerol and its key fraction, free fatty acids. Bio-oils have been obtained from liquefaction processes at 340 degrees C. They were subjected to various physico-chemical characterization methods. FTIR data indicated a reduction in aliphatic structures and an increase in more oxidized and, probably, more polycondensed aromatic components resulting from the addition of crude glycerol to swine manure. GC-MS data indicated that the addition of crude glycerol facilitated the esterification reaction in sub-critical water to convert organic acids contained in bio-oil into various kinds of esters. The dynamic viscosity of bio-oil decreased dramatically by adding crude glycerol into the swine manure. (C) 2010 Elsevier Ltd. All rights reserved.