One-pot acetalizations of cyclohexanone, acetophenone and benzophenone were carried out using methanol over H-montmorillonite clay (a mesoporous material), silica, alumina, and different zeolites such as HFAU-Y, HBeta, H-ZSM-5, and H-mordenite. In all the cases, a single product-the corresponding dimethylacetal-was obtained in high yields. Hemiacetal formation was not observed with any catalyst. A comparison of catalytic activity indicated that montmorillonite K-10 is the most active catalyst for the reaction. As evidenced by the reaction time studies, the catalyst decay is greater over the zeolite catalyst than over the clay. (C) 2004 Elsevier B.V. All rights reserved.