NaX zeolite loaded with potassium nitrate was prepared, characterized and applied as heterogeneous catalyst in the transesterification of sunflower oil with methanol. The influence of the loaded amount of KNO3, the methanol/oil molar ratio and catalyst amount on the final methyl esters content was studied. The catalysts were characterized by atomic absorption spectrometry (AAS), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and CO2 temperature programmed desorption (CO2-TPD). The results demonstrated that methyl esters content was highly influenced by basicity and the catalyst with the highest catalytic activity was obtained using an amount of 35 wt.% KNO3. The produced biodiesel was 98.2% methyl esters when the transesterification reaction was carried out at atmospheric pressure, a reaction temperature of 60 degrees C, a reaction time of 6 h, with a 21:1 M ratio of methanol to sunflower oil, and a catalyst amount of 10%. The biodiesel obtained at these conditions met the European norm EN-14214 regarding viscosity and acid value. (C) 2012 Elsevier Ltd. All rights reserved.