The acetone-sensitized irradiation using UV-B (ultraviolet light, 280-320 nm; sunlamps) of thymidylyl(3'-->5')deoxyfluorouridine monophosphate produces two main photoproducts. The distribution of these photoproducts is dependent on the pH of the irradiation solution. At pH 6, the cis-syn cyclobutane-type photodimer is the major product, whereas at high pH (8-10) a photoadduct is the major product. These photoproducts have been identified and structurally characterized by H-1 and C-13 NMR spectroscopy. The photoadduct arises from defluorination of the 5-fluorouracil moiety. The structure of the photoadduct maintains the sugar-phosphate backbone of the starting material (d-TpF), and contains a saturated thymine moiety with an added Thy(C6-hydroxyl) and a Thy(C5)-(C5)Ura covalent bond.