Disproportionation of 1,2,4-trimethylbenzene was carried out at 200 degrees C to investigate the effect oi hydrogen spillover on the stabilization of the catalytic activities of ultrastable Y (USY) zeolite and pillared montmorillonite. The catalytic activity of pillared montmorillonite, the acidity of which is mainly due to Lewis acid sites, became stable against deactivation in the presence of spiltover hydrogen. There was a less pronounced effect of spiltover hydrogen on the deactivation of USY catalyst treated at 400 degrees C, When the surface acidity of USY was converted from Bronsted to Lewis acidity by treatment at temperatures as high as 750 degrees C, however, the deactivation was effectively suppressed by spiltover hydrogen. We concluded from these results that the stabilization of activity by spiltover hydrogen is linked to Lewis acidity, irrespective of the type of catalyst.