SWy-2 (Wyoming), Sap-Ca saponite (California) and SAz-1 (Cheto, Arizona) were exchanged with different cations (Al, Ni, Mg, Ca and Na). The catalytic activity of these ion-exchanged clays was measured directly using the isomerization of α-pinene at 80°C for 2 h to yield camphene, limonene and other minor products. The order of activity for the different cation-exchanged forms was Al > Ni > Mg > Ca > Na, which correlated well with the known polarizing power of these cations and the resulting interlayer acidity. Catalysts derived from Sap-Ca were the most active followed by SWy-2, with SAz-1 the least active by a considerable margin. The greater activity of Sap-Ca over SWy-2 was attributed to the high tetrahedral charge in the former. Reduction of the layer charge of SAz-1 using Li fixation caused a significant improvement in the catalytic activity of the Al-exchanged reduced-charge SAz-1.