Transesterification of triacetin, tributyrin and soybean oil with methanol was examined over rehydrated hydrotalcites (HT-reh), which were previously calcined at 550 degrees C and exposed to an ambient atmosphere, in order to investigate the effect of the water content on their catalytic activity. Rehydration of a calcined hydrotalcite at 25 degrees C produced uniform mesopores of around 4 nm diameter, while the removal of water from the HT-reh by pretreatment at 500 degrees C, formed large heterogeneous mesopores. Methanol was adsorbed and desorbed rapidly in the mesopores of the HT-reh filled with water. The pretreatment of the HT-reh at 500 degrees C removed both water and carbon dioxide, producing strong basic sites. The high activity of fully rehydrated hydrotalcite in the transesterification of three triglycerides was explained by the generation of Bronsted basic sites and the rapid migration of methanol in the mesopores, while the improved activity of the rehydrated hydrotalcite pretreated at 500 degrees C by the generation of strong basic sites and empty mesopores due to the removal of water and carbon dioxide. (C) 2010 Elsevier B.V. All rights reserved.