The effect of Pd loading on the catalytic activity of montmorillonite pillared with alumina (Al-mont) was investigated. The catalyst deactivation was prevented by loading a small amount of Pd on Al-mont and using H 2 as a carrier gas, although the initial activity was reduced. Since the mixing of 0.5 wt% Pd Al 2O 3 with Al-mont gave similar effects to those from the Pd loading, the stabilization of activity in the presence of both Pd and H 2 may be caused by spilled-over hydrogen. By comparison, the activity of H-Y was not stabilized under the same condition. The strong acid sites of Al-mont, which were mostly Lewis acidic, decreased by Pd loading in the atmosphere of H 2 stream, whereas those of H-Y changed little, due to Brönsted acidity. These results suggest that Lewis acid sites can play a very important role in the stabilization of the catalytic activity by spilled-over hydrogen.