Mesoporous silicas were functionalized with phenyl groups and were used as solid acid catalysts in acetalization of aldehydes and ketones with ethylene glycol or methanol. It was shown that cyclohexanone as well as aromatic and linear aldehydes could be converted to the corresponding dimethylacetals in high conversion and selectivity over the catalyst. They also performed well in the isopropylation reactions of naphthalene with isopropanol.