Methyl esters produced by transesterification of vegetable oils are the main components of biodiesel, an alternative, biodegradable, non-toxic biofuel produced from renewable resources. The use of solid catalysts allows the development of environmentally friendly processes to produce biodiesel. In this work, Zn,Al-mixed oxides with different Al/(Al + Zn) molar ratios were studied as catalysts for the transesterification of soybean oil with methanol. TPD-CO2 (temperature programmed desorption of CO2) and model reactions such as retroaldolization of diacetone alcohol and 2-propanol transformation were employed to determine the basic properties of the catalysts. The catalytic activity was influenced by the chemical composition of the catalyst and showed a good correlation with basic site density determined by TPD-CO2. The RSM (response surface methodology) was used to select experimental conditions such as alcohol/oil molar ratio, reaction temperature, and catalyst loading that maximize fatty acid methyl esters yield. The best experimental conditions indicated by RSM were reaction temperature of 182.5 degrees C, catalyst loading of 5.0 wt.%, and alcohol/oil molar ratio of 45, that provided a biodiesel yield of 86% without significant leaching of the catalyst. (C) 2014 Elsevier Ltd. All rights reserved.