The photochemistry of the dinucleoside monophosphate thymidylyl-(3′-5′)-5-methyl-2′-deoxycytidine (Tpm⁵dC) has been studied in aqueous solution using both 254 nm and UV-B radiation. A variety of dinucleotide photoproducts containing 5-methylcytosine (m⁵C) have been isolated and characterized. These include two cyclobutane dimers (CBD) (the cis-syn [c,s]and trans-syn forms), a (6-4) adduct and its related Dewar isomer, and two isomers of a product in which the m⁵C moiety was converted into an acrylamidine. Small amounts of thymidylyl-(3′-5′)-thymidine (TpT) were also formed, presumably as a secondary photoreaction product. In addition, a photoproduct was characterized in which the m⁵C moiety was lost, thus generating 3′-thymidylic acid esterified with 2′-deoxyribose at the 5-hydroxyl on the sugar moiety. The c,s CBD of Tpm⁵dC readily undergoes deamination to form the corresponding CBD of TpT. The kinetics of this deamination process has been studied; the corresponding enthalpy and entropy of activation for the reaction have been evaluated at pH 7.4 as being, respectively, 73.4 kJ/mol and −103.5 J/K mol. Deamination was not observed for the other characterized photoproducts of Tpm⁵dC.