Mesoporous Si-MCM-41 and Al-MCM-41 molecular sieves in four Si/Al ratios: 25, 50, 75 and 100, were synthesized under hydrothermal conditions. Ten weight percent, 15 wt.% and 20 wt.% H3PW12O40 (PW) was supported on Si-MCM-41. They were characterized using powder Xray diffraction (XRD), FT-IR, BET and TEM. The catalytic activity of these materials were tested for the acetalization of carbonyl compounds with 2,2-bis(hydroxymethyl)propane-1,3-diol at refluxing temperature using a Dean-Stark apparatus. The hydrophobic properties of catalyst and alcohols, and the steric properties of carbonyl compounds were suggested to play important roles in the acetalization. Fifteen weight percent PW/ MCM-41 was found to be more active than other catalysts used and to have similar activity to that of HPA. TEM studies show that the high dispersion of HPA units, which was maintained through out the reaction, was lost when recycled even for shorter reaction cycle. The reaction was also studied over commercially available heteropolyacids, and zeolites (H beta, HY, HM and ZSM-5). Occurrence of the reaction mainly within the pores was confirmed by running the reaction over the HPA loaded on an as-prepared catalyst that provides less product yield than the calcined material. (c) 2005 Published by Elsevier B.V.