Montmorillonites ion-exchanged with Li (super +) , Na (super +) , K (super +) , Ca (super 2+) , Mg (super 2+) and Ni (super 2+) and acidic clay were used as catalysts for the polymerization of a cyclic siloxane monomer, 2,4,6,8-tetramethylcyclotetrasiloxane. Montmorillonites with Ni (super 2+) and Mg (super 2+) in the interlayer, and acidic clay exhibit a greater ability for siloxane polymerization in both yield and mean molecular weight of products than those containing Li+, Na (super +) and K (super +) . The difference in catalytic ability of the ion-exchanged montmorillonites is caused by the number of Bronsted acid sites due to the polarization of H 2 O. This was confirmed experimentally by FTIR analysis of pyridine-treated samples. Therefore, it may be possible to design a catalyst for controlling siloxane polymerization (i.e. mean molecular weight of product) by changing the number of Bronsted acid sites through exchange of the interlayer cations of montmorillonites.