This work investigates the use of MgAl and MgCa oxides as catalysts for transesterification processes, in order to evaluate their potential as heterogeneous catalysts for biodiesel production. The catalysts were characterized by XRD (X-ray diffraction), XPS (X-ray photoelectron spectroscopy), SEM (Scanning electron microscopy), CO2-TPD and N-2 adsorption. MgCa oxides were more active than MgAl in the transesterification of ethyl butyrate with methanol at 60 degrees C and atmospheric pressure. The highest activity was found for a bulk Mg:Ca molar ratio of 3.8, with a conversion of 45%, whereas MgO was inactive. Moreover, lixiviation of the active phase was not observed, thus excluding the contribution of the homogeneous catalysis to the studied transesterification process. in the methanolysis of sunflower oil, the activity follows the same tendency observed in the transesterification of ethyl butyrate with methanol. A FAME (fatty acid methyl esters) yield of 92% has been reached with a methanol:oil molar ratio of 12, a reaction temperature of 333 K and a 2.5 wt.% of the Mg:Ca(3.8) catalyst. (C) 2008 Elsevier B.V. All rights reserved.