The pyrimidine nucleobases contained in DNA undergo a variety of photoinduced reactions in which two moieties become joined to form a product (e.g. formation of cyclobutane dimers and [6-4] adducts). Herein, we describe a new type of photoconjugation reaction that has been shown to occur for 5-methylcytosine (5-MeC), 1,5-dimethylcytosine (1,5-diMeC), 1-methylthymine and thymidine; in this reaction the 5-methyl group of one nucleobase (or nucleoside) becomes attached to the 4-position of the second moiety. For example, 5-MeC forms α-4′-(5′-methylpyrimidin-2′-one)-5-methylcytosine. The various (α-4) conjugates are produced upon irradiation of the parent compound in frozen aqueous solution at −78.5°C. The UV spectra of these compounds display a characteristic “double humped” profile, similar to that expected from overlaying the spectrum of parent nucleobase with that of a 2′-pyrimidone moiety. Preliminary results suggest that thymine and 5-methyl-2′-deoxycytidine (5-MedCyd) form analogous photoproducts. A variety of other previously unreported photoproducts are described as well for the 5-MeC, 1,5-diMeC and 5-MedCyd systems.