A catalytic composite of a refractory inorganic oxide whose bound surface hydroxyl group has reacted with a Friedel-Crafts type metal halide and whose acidity has been modified by the deposition of a monovalent metal cation, especially an alkali metal cation, or alkaline earth metal cation shows superior selectivity and a significantly decreased cracking tendency relative to similar catalysts without the monovalent metal or alkaline earth metal cation. The resulting alkylate from an isobutane-butene feedstock shows a substantial increase in research octane number relative to the alkylate formed by a similar catalyst which has not been so modified by an alkali or alkaline earth metal cation.