It has been recognized for some time that the incorporation of fluorine in oxide catalysts (for example, alumina, silica-alumina, or zeolites) enhances their activity for acid-catalyzed reactions such as cracking, isomerization, alkylation, polymerization, and disproportionation. These reactions are thought to proceed via carbocation intermediates which are formed and stabilized on surface protonic sites. The incorporation of fluorine increases the activity by enhancing the acidic properties of the catalyst. Fluorine incorporated in an oxide catalyst replaces surface O or OH, and because fluorine is very electronegative, it polarizes the lattice more than the groups it replaces, and this increases the acidity of both protonic (Brönsted) and nonprotonic (Lewis) sites on the surface. As will be seen, pure alumina is inactive or only slightly active for acid-catalyzed reactions. In contrast, it has been shown repeatedly that fluorinated alumina is a very active, selective, and stable catalyst for such reactions. The formation of fluorinated solid “superacids” which are active catalysts at low temperatures also has been reported. Very recently fluorination has been used in the modification of zeolite catalysts for better activity.