The synthesis of diethyl phthalate (DEP) from phthalic anhydride (PAH) and ethanol (EtOH) has been carried out successfully on aluminum-, iron- and zinc- containing MCM-41 molecular sieves. These catalysts were prepared by the hydrothermal method. The catalysts have been characterized by the XRD, BET (surface area) and Si-29 and, Al-27 MAS-NMR techniques. The catalytic activity of these molecular sieves has been tested by the above-mentioned reaction, by the batch processes at 135, 150 and 175 degrees C. Both conversion and selectivity increased with an increase in temperature and mole ratio (EtOH/PAH). The activity of these catalysts followed the order: Al-MCM-41 (Si/Al = 112) > Fe-MCM-41 (Si/Fe = 115) > Al-MCM-41 (Si/Al = 70) > Al-MCM-41 (Si/Al = 52) > Fe-MCM-41 (Si/Fe = 61) > Al, Zn-MCM-41 (Si/Al + Zn = 104) > Al-MCM-41 (Si/Al = 30). The reaction yielded both monoethyl phthalate (MEP) and diethyl phthalate (DEP). The nature of the catalyst sites has been clarified using water as an impurity. The weight of the catalyst was optimized at 0.06 g. Some times hydrophilic and some times hydrophobic nature of the catalyst been explained by the influence of water; the external surface acidity also facilitates the reaction. This has been confirmed by the addition test reaction. (c) 2006 Published by Elsevier B.V.