Sulfated metal oxides (SMO) were able to catalyze the reaction of carbonyl compounds with ethylene glycol (EG), producing cyclic acetals at near ambient temperature. Sulfated hafnium oxide containing only Lewis acid sites (indicated by IR/pyridine adsorption experiments) was found active for the reaction. However, these Lewis acid sites might not be the active sites for the reaction, because they were most likely transformed into Bronsted acid sites by water produced in the reaction. Both electronic and steric effects played important roles in this reaction. The sulfate group coordinating to the active acid site had exerted a steric influence on the reaction. Therefore, the less bulky cyclic acetals were more readily formed. The carbonyl group conjugated with a double bond delocalized the positive charge on the carbocation reaction intermediate, resulting in lower cyclic acetal yields.