Novel codimers (I) of norbornadiene and phenylacetylenes and their hydrogenated derivatives (II), having the following structures ##STR1## wherein R is a hydrogen, a phenyl, or a phenyl having alkyl substitutents and R.sub.1 is a phenyl or a phenyl having alkyl substituents and processes for preparing both are disclosed. Codimer (II) can be used as a high energy fuel or a diluent for such a fuel. Process for making codimer (I) involves reacting norbornadiene and phenylacetylene or diphenylacetylene with a catalyst system of cobaltic or cobaltous acetylacetonate, 1,2-bisdiphenylphosphino ethane and an alkyl aluminum chloride.