Several Mg/Al mixed oxides, prepared by three different co-precipitation procedures, have been tested in different transesterification reactions. The catalytic activity for sunflower oil transesterification correlates with the leachable basicity, and not with the total basicity, measured by titration methods. The presence of residual alkaline ions after crystallization is the main source for the strong basicity in this kind of solids, and the difficulty in the complete elimination of the alkaline traces are probably the origin of the strong discrepancies in basicity detected along the literature data. The effect of these alkaline traces is more evident with a more demanding reaction, such as that of methyl palmitate with isobutanol. In such case the alkaline-free solids did not show any catalytic activity. The nature of these strong basic sites makes the recovery problematic, given that part of the alkaline cation is lost to the reaction medium, a process favored by an increase in the polarity of the medium. This fact is also a general limitation for the application of this kind of solids to liquid phase reactions requiring high basicity. (C) 2009 Elsevier B.V. All rights reserved.