Mg-Zr (Mg/(Mg + Zr) = 0.5, nominal atomic ratio) catalysts have been synthesized following two different preparation methods (co-precipitation and alcogel) and tested in furfural/acetone aldol condensation reactions. The untreated co-precipitated solid shows the presence of Mg(OH)(2) and Zr oxyhydroxide-like phases, which are not very active in the furfural/acetone aldol condensation. When the solid is treated in O-2/Ar flow at increasing temperatures (up to 873 K), the initial phases evolve to a mixture of cubic MgxZr1-xO2-x mixed oxide and MgO. The latter phase (cubic MgO) shows a significantly enhanced catalytic activity. In contrast, the same species are already generated in the solid obtained in the alcogel route. Nevertheless, this latter preparation method also requires a high temperature treatment step to form highly active catalysts in furfural/acetone aldol condensation because the ethoxy residues originated during aerogel preparation need to be removed. (C) 2010 Elsevier B.V. All rights reserved.