In this work, response surface methodology was used to optimize the conditions for the transesterification of sunflower oil with methanol, using Na2O supported on NaX faujasite zeolite as catalyst. By experimental design a quadratic polynomial equation was obtained for biodiesel yield. Multiple regression analysis and verification experiments confirmed the validity of the predicted model. The studied variables were reaction temperature (40,50 and 60 degrees C), catalysts weight base oil (5, 7.5 and 10 wt%), Na content (5.5, 7.5, 10 and 11.5 wt%) and methanol: sunflower oil molar ratio (4:1,5:1 and 6:1). Results from the optimization study showed that reaction temperature and molar ratio methanol:oil can be optimized. Catalyst characterization was carried out by scanning electron microscopy, atomic absorption, X-ray photoelectron spectroscopy, nitrogen adsorption/desorption and basicity determination by Hammett method. Biodiesel with highest methyl esters content (99.3%) was obtained with the catalyst with 10% Na content, at 60 degrees C, 6:1 methanol:sunflower oil molar ratio and 10 wt% catalyst concentration. The produced biodiesel was found to fulfill the specifications of the European Norm UNE-EN 14214 regarding viscosity, flash point and acid value. (C) 2013 Elsevier B.V. All rights reserved.