In order to prepare solid Lewis acid catalysts, γ-alumina was first dehydrated at 773 K and was then exposed to chlorine gas for 1 h at 773–1373 K. The aluminas chlorinated above 973 K had no hydroxyl group and, in the adsorption of pyridine, yielded the typical and enhanced four IRabsorption peaks assigned to pyridine coordinated on Lewis acid sites. The absence of b was ascertained by IR spectroscopic studies of adsorbed carbon dioxide and by the absence of the formation of allyl alcohol and acrolein in propylene oxide isomerization. With an increase in chlorination temperature, the intensity of the 1628 cm −1 peak of coordinated pyridine decreased from a maximum at 973 K, but the difference in the peak intensity between the chlorinated and dehydrated aluminas increased. A similar temperature dependence was obtained in the chlorine content. Further, from ESCA data, the chlorine on the treated alumina was concluded to exist in the form of a chloride ion linked to an aluminium cation. Consequently, the enhanced Lewis acid sites, which changed the yellow colour of 2,4-dinitrofluorobenzene (p K a = −14.52) to yellow-orange, were concluded to be induced by the chloride ions. In addition, the chlorinated alumina adsorbed 1-chloropropane to form π-allyl and isopropyl cations simultaneously, in addition to the propyl cation, and yielded a high cis/trans ratio of 6.2 in 1-butene isomerization at 273 K.