The obtained heats of combustion were 46.65 +/- 0.20; 38.09 +/- 0.31; 39.67 +/- 0.22; 41.88 +/- 0.31; 37.86 +/- 0.46; 39.89 +/- 0.09; 41.28 +/- 0.31 MJ/kg for diesel, transesterified soybean oil, pyrolysed soybean oil and crude soybean oil, transesterified palm tree oil (Elaeis sp.), pyrolysed palm tree oil, crude palm tree oil, respectively. The results show the heats of combustion of biofuels are approximately 17% smaller than traditional diesel. The data also show the heats of combustion depend on the methodology used for the biofuel production. Addition of biofuels to traditional diesel fuel results in a linear decrease of the heat of combustion with the amount of the alternative fuel added to the diesel, However, for blends with 5% biofuels, which is the limit demanded by Brazilian legislation, no significant decrease of the heat of combustion of the commercial diesel was observed. (c) 2006 Elsevier B.V. All rights reserved.