SummaryAdsorbed aromatic compounds were relatively easily oxidized. As active intermediates, radical cations occurred which, in carcinogenic compounds, preferred electrephilic substitutions to further oxidation. Benzo[a]pyrene in an adsorbed state could be oxidized by iodine in the presence of pyrimidines and purines. Some of the many products formed by adenine and purine were isolated and investigated spectroscopically. A 7- (or 9-) benzo[a]pyrenylpurine formed, and 6-aminobenzo[a]pyrene was a degradation product. The significance of oneelectron oxidation as a possible primary step in chemical carcinogenesis is discussed.