The protection of aromatic aldehydes with acetic anhydride was investigated in the catalytic presence of acid activated four alkylphosphonium exchanged montmorillonites (Px-MMT). The acid sites of all catalysts were investigated by infrared spectroscopy and thermal analysis using pyridine as a molecular probe. The yield of reaction is roughly related to the strength of acid sites and depends also on the substitution on the benzene ring of the aldehydes. The order of activity of selected organocation-exchanged forms was P16>P28>P32∼P40 which correlates well with the size of these cations and resulting acidity. P32-MMT is revealed to be an effective catalyst for protection of aromatic aldehydes with Ac2O and produces almost solely 1, 1-diacetates.