Dimethylacetals have been prepared by reacting aldehydes with trimethyl orthoformate, Large pore zeolites (HY and beta) as well as mesoporous silica-aluminas with regular pore sizes, such as MCM-41 and SAM, and commercial amorphous silica-alumina with a broader distribution of pore sizes have been used as catalysts. The results indicate that in addition to bridging hydroxyl groups, silanols can also be active for carrying out this reaction. Zeolites are more active than the mesoporous materials, but their activity decays faster. By carrying out the reaction with aldehydes of different molecular sizes it is shown that the MCM-41 mesoporous materials are promising catalysts for their use in the production of fine chemicals which involve bulky reactants or products. Finally, mesoporous catalysts with a very narrow pore size distribution show a better catalytic behavior than others with a broader pore size distribution.