The acetal formation mechanism under acid-free Rh-catalyzed hydroformylation-acetalization condition has been studied using different rhodium catalyst precursors in MeOH. In the absence of added acidic co-catalyst, the acetalization is catalyzed by the H+ formed in situ under hydroformylation condition, and Rh active site on Rh-phosphine catalyst did not exhibit catalytic activity for acetalization. Whether H+ can be generated in situ is related with the structure of rhodium catalyst precursor. Under hydroformylation condition, added Bronsted acids as co-catalysts can improve acetalization efficiency, but the H+ concentration in the system should not be excessively high to avoid the acid-induced inhibition for hydroformylation.