In a previous study, waste fish oil was converted into bio-oil by a fast pyrolysis process at 525 degrees C in a continuous pilot plant reactor with 72-73% yield. The bio-oil was distilled to obtain light bio-oil and heavy bio-oil and these biofuels were characterized in terms of their physico-chemical properties. In this study, the chemical composition of light bio-oil and heavy bio-oil was determined using GC-FID, GC-MS, (1)H and (13)C NMR techniques. The GC-MS analysis of waste fish oil showed the main composition of fatty acids to be the following: C(16:0) (15.87%), C(18:2) (20.96%), C(18:1) (17.29%), C(20:5) (5.11%), C(20:1) (7.59%), C(22:6) (4.53%), C(22:1) (10.42%) and others. The GC-FID analysis of the light bio-oil showed 482 compounds that were PIONA classified as paraffins (4.48%), iso-paraffins (8.31%), olefins (26.56%), naphthenes (6.07%) and aromatics (16.86%). The heavy bio-oil had a similar chromatographic profile as diesel oil, with a high content of carboxylic acids and olefins. These results are in good agreement with those for the gasoline and diesel oil fractions of petroleum. (C) 2009 Elsevier Ltd. All rights reserved.