Isopyrimidines are formed as a result of the oxidation of the 6-yl radicals derived by ˙OH attack on pyrimidines and dihydropyrimidines. The Kinetics of the rearrangment of the isopyrimidines into the corresponding pyrimidines has been followed by pulse radiolysis. The rearrangement of isouracil into uracil is proton-catalysed (k 1.8 × 10⁷ l mol^–1 s^–1). Around pH 7 a spontaneous reaction, k 3 000 s^–1, is observed. On increasing the pH the isouracil deprotonates at N(3)(pK_aca. 9.4). The spontaneous rearrangement of the isouracil anion is considerably slower (k≤ 50 s ¹). At pH > 10.5 an OH^–- catalysed reaction sets in (k 4.9 × 10⁵ l mol¹ s^–1) which involves a second deprotonation, at C(5). Similar results have been obtained for the rearrangement of 5-hydroxyisouracil into isobarbituric acid. On blocking the N(3) position as in 3-methylisouracil, the OH^–-induced rearrangement sets in at a much lower pH (pH ≤ 9.5), i.e. the rearrangement is faster (k 2.7 × 10⁷ l mol^–1 s^–1) than that observed in the other two systems.