The diphenylmethylenecyclohexane,2, and the substituted 2-methyleneadamantanes,3-7, have been prepared. The radical cation of each was generated by photochemical oxidation using 1,4-dicyanobenzene as the sensitizer, and their reactivity was examined in methanol-acetonitrile mixtures with added tetraethylammonium cyanide. Although compound2reacted only by tautomerization, the other compounds all gave addition products. For the monoaryl alkenes3,4,and5, comparable yields of both methanol and HCN addition products were obtained, resulting from nucleophilic attack at either end of the alkene radical cation. For the diphenylalkenes6and7, the regioselectivity favored nucleophilic attack at the adamantyl carbon, and the addition products were predominantly those of HCN addition. For the 5-methoxy compound7, HCN addition was facially selective with asyn:antiratio of 58:42.Key words: methyleneadamantanes, facial selectivity, radical cations.