Mesoporous Al-MCM-41 molecular sieves in the Si/Al ratios 25, 50, 75 and 100 were synthesized under hydrothermal condition. They were characterised using powder X-ray diffraction (XRD), FT-IR and BET surface area analyser. The catalytic activity of these materials was tested for the slurry phase acetalization of cyclohexanone with methanol (MET), ethylene glycol (EG) and pentaerythritol (PEN) at 40-120 degrees C. MET conversion was found to be higher than that of EG and PEN. Hydrophobic property of catalyst and alcohols and also the steric properties of alcohols were suggested to play an important role in the acetalization. For MET and EG, Al-MCM-41 (100) was found to be more active than other catalyst but for PEN, Al-MCM-41 (25) was found to be more active. The reaction was also studied over H3PW12O40 supported MCM-41, commercially available heteropolyacid (H(3)PW(12)O(40)(.)nH(2)O) and zeolites (HM, H beta and ZSM-5). Zeolites showed less activity for EG, PEN but comparable activity to MET. The requirement of Bronsted acid sites for the reaction was clearly established by running the reaction in the absence of catalyst. (c) 2006 Elsevier B.V All rights reserved.