Norbornadiene [bicyclo(2.2.1)heptadiene-2,5] is dimerized to the exo-exo stereoisomer of the hexacyclic dimer of norbornadiene at both an excellent selectivity and conversion using an effective amount of a three component catalytic system of diethylaluminum chloride, ferric or ferrous or cobaltic or cobaltous acetylacetonate and bis(1,2-diphenylphosphino)ethane. The reaction rate is rapid. After hydrogenation the exo-exo hexacyclic dimer can be used as a component of high energy fuel for either jet or rocket propulsion.