Sulfated metal oxides including SO42-/ZrO2, SO42-/TiO2, SO42-/HfO2, SO42-/Fe2O3, SO42-/SnO2, and SO42-/Al2O3 were highly efficient catalysts for the reaction of aldehydes and ketones with trimethyl orthoformate producing dimethyl acetals under mild reaction conditions. At room temperature, dimethyl acetal yields of 83-100% were obtained for the five carbonyl compounds chosen. These mesoporous solid acids also effectively catalyzed the hydrolysis of dimethyl acetal to regenerate the original carbonyl compounds in aqueous acetone. They not only provided an effective method for synthesizing dimethyl acetals of larger molecular size but also acted as a versatile catalyst for protecting and deprotecting carbonyl groups during organic synthesis.