A method for producing adamantane includes isomerizing exo-tetrahydrodicyclopentadiene into adamantane in an acidic chloroaluminate ionic liquid composed of aluminum(III) trichloride and a quaternary ammonium halide having a quaternary ammonium cation selected from a group consisting of tetraalkylammonium, dialkylpyridinium, and trialkylimidazolium.