The steady state photolysis of 5-bromouracil (BU) in aqueous solution has been studied as a function of wavelength, pH, temperature, and hydrogen-donor concentration. Under all conditions studied, the primary reaction is shown to be C-Br bond cleavage followed by abstraction from the hydrogen-donor to give uracil and HBr. At pH > 12 further products are formed. In deoxygenated aqueous solution at pH 6, 20°C, and 254 nm, the quantum yield of BU consumption, Φ (-BU), is 1.8 × 10 -3 independent of hydrogen-donor type or concentration (e.g. 3 × 10 -2 to 2 M MeOH). With increasing pH, Φ (-BU) increases stepwise to 0.012 at pH 10 and to 0.28 at pH 14. pK-values calculated from these data are the same as ground state pK values. Φ (-BU) increases with temperature with an activation energy of approx. 3.4 kcal/mol. Φ (-BU) increases with photon energy. Above 2 M MeOH Φ (-BU) increases reaching Φ (-BU) = 0.025 in neat MeOH. Similar high, solvent dependent, values are obtained for other hydrogen-donor solvents. In neat organic solvents an additional reaction with BU induced by solvent radicals was observed. These results have been explained in terms of a homolytic dissociation of the C-Br bond of the excited BU followed by recombination or H atom abstraction by the radicals. At high hydrogen-donor concentration H atom abstraction can compete with cage recombination. A comparison has been made between BU photolysis in organic, hydrogendonor solvents and BU photolysis within the DNA of bacteria or phages. It has been concluded that the much higher quantum yields observed for chain breaks in the photolysis of DNA containing BU compared to photolysis of BU in aqueous solution is due to the high local concentration of hydrogen-donors (sugar molecules) within the DNA molecule, even in dilute aqueous solution.