It has been found that the (6-4) photoproduct of thymidylyl-(3'----5')-deoxycytidine (TpdC) is converted quantitatively to a further photoproduct upon exposure to Pyrex-filtered medium pressure mercury arc light. Infrared UV, FAB MS, 1H NMR, 13C NMR and 31P NMR spectra were obtained for both the (6-4) product and its photolysis product. 1H NMR assignments were made on the basis of proton decoupling and homonuclear shift correlated experiments and 13C NMR assignments were made on the basis of proton-detected heteronuclear shift correlated experiments. The Dewar pyrimidinone structure was assigned to the photolysis product by analysis of the spectral data in comparison to those of the Dewar photoproduct of TpT and other Dewar pyrimidinones. The (6-4) product of TpdC is the second member of the class of (6-4) photoproducts that has been found to photoisomerize to its Dewar valence isomer upon exposure to wavelengths greater than 280 nm, the first being that of TpT (Taylor and Cohrs, 1987, J. Am. Chem. Soc. 109, 2834-2835). These results further support the proposal that all members of the (6-4) photoproduct class are converted to their Dewar valence isomers upon exposure to sunlight.