The catalytic activity of an aluminum complex/silanol system was found to vary with the aluminum ligand as follows: β‐diketonate < β‐ketoester anion < ortho‐carbonylphenolate. Aluminum complexes with electron donating groups conjugated with a carbonyl group had higher catalytic activity. The catalyst activity correlated with the strength of interaction between the aluminum complex and the silanol; there were two such interaction mechanisms. Cyclohexene oxide was polymerized cationically by the activated proton of SiOH. The activated proton is considered to be similar to Brönsted acid on silica‐alumina catalyst; therefore, the catalyst would be a model of the silica‐alumina catalyst.