This study optimized the conversion of animal fat wastes into ethylic biodiesel by alkali-catalyzed process under mild conditions. A mix of chicken and swine fat residues was used as feedstock for biodiesel production. A full 3(3) factorial design was used to optimize process parameters for maximum fatty acid ethyl esters yield. Factors were evaluated at three different levels: temperature (30; 50; 70 degrees C), ethanol: fat molar ratio (6: 1; 7: 1; 8: 1) and catalyst concentration (0.44; 0.88; 1.32 wt.%). Effects of the process variables were analyzed using response surface methodology. Moreover, optimum conditions were applied in a bench-scale reactor and biofuel produced was characterized. It was observed that at high temperatures (50 and 70 degrees C), phase separation between biodiesel and glycerol was impaired. Although high conversion was achieved (96.2%) at 70 degrees C, this condition is not recommended because no spontaneous phase separation was verified. On the other hand, 30 degrees C was identified as the best temperature for biodiesel ethanolysis, using 0.96 wt.% catalyst and 7: 1 ethanol: fat molar ratio. With these conditions, it is possible to achieve around 83% conversion. Despite the oxidative stability and total glycerin, biodiesel measured properties agreed with quality requirements established by Official Regulations (ASTM 6751 and EN 14214). (C) 2012 Elsevier Ltd. All rights reserved.