UV Photosylis (254 nm) of 5-iodouracil in aqueous, oxygenated solution in presence of KI, FeSO4, KBr and K2SO4 was investigated over a pH-range of 1—6. Uracil, isodialuric acid, and isobarbituric acid were identified as photoproducts. At a constant pH-value (pH=1) at high KI-concentrations (10-1 mole/1) the main product is uracil. With decreasing KI-concentrations the yield of uracil decreases and isobarbituric acid appears as a photoproduct. On further decreasing KI-concentrations (< 10-2 mole/1) the yield of isobarbituric acid decreases and the formation of isodialuric acid commences. UV Photolysis of iodouracil at constant KI-concentrations (10—1 mole/1) and increasing pH (pH = 1—6) yields similar results with one exception. At higher pH-values on account of isodialuric acid at least four further compounds are formed. The yield of these products increases with increasing pH value. Addition of KBr and K2SO4 has no effect on the product distribution. The possible mechanisms for the formation of the reaction products are discussed.