Al-MCM-41 molecular sieves (Si/Al = 50, 100, and 150) were synthesized hydro-thermally and characterized by XRD, FTIR, BET (surface area) and Si, Al MAS-NMR techniques. The catalytic activities of these molecular sieves and of Hp-zeolite were tested for esterification of maleic anhydride in the liquid phase at 80, 100 and 150degreesC. The reaction was observed to yield monomethylmaleate even in the absence of catalyst, as the product is strain-free, whereas the formation of dimethylmaleate required the presence of a catalyst. Among the catalysts, Hbeta was found to be more active than Al-MCM-41 molecular sieves. The lower activity of Al-MCM-41 catalyst was due to pore blocking by reactants as well as by products. An interesting observation in this study is that dimethylmaleate isomerisation to dimethylfumarate is not favored. At 80degreesC this isomerisation was completely absent over Al-MCM-41 molecular sieves, and was less than 1% over Hbeta. The complete conversion of maleic anhydride with methanol to monomethylmaleate stands as a discovery that will help to design new routes to synthesize a wide variety of new heteroesters. (C) 2004 Elsevier B.V. All rights reserved.