Abstract— Ultraviolet light irradiation of DNA in the presence of alcohols such as 2‐propanol leads to hydroxyalkylation of the purine bases. The purine photoproducts were identified as C‐8 hydroxyalkyl derivatives of adenine and guanine by chromatographic mobility, co‐crystallization with authentic samples, and by photoreversal to the original purines. Experiments are reported which demonstrate that the photoalkylation reaction is dependent upon secondary structural alterations in DNA. It is suggested that the photoalkylation of purines proceeds through free‐radical intermediates. The presence of alcohols during irradiation considerably decreases pyrimidine‐dimer formation.