Although uric acid is known to react with many reactive oxygen species, its specific oxidation products have not been fully characterized. We now report that 5- N -carboxyimino-6- N -chloroaminopyrimidine-2,4(3 H )-dione (CCPD) is a hypochlorite (ClO⁻)-specific oxidation product of uric acid. The yield of CCPD was 40–70% regardless of the rate of mixing of ClO⁻ with uric acid. A previously reported product, allantoin (AL), was a minor product. Its yield (0–20%) decreased with decreasing rate of mixing of ClO⁻ with uric acid, indicating that allantoin is less important in vivo . Kinetic studies revealed that the formation of CCPD required two molecules of ClO⁻ per uric acid reacted. The identity of CCPD was determined from its molecular formula (C₅H₃ClN₄O₄) measured by LC/time-of-flight mass spectrometry and a plausible reaction mechanism. This assumption was verified by the fact that all mass fragments ( m/z −173, −138, −113, and −110) fit with the chemical structure of CCPD and its tautomers. Isolated CCPD was stable at pH 6.0–8.0 at 37°C for at least 6 h. The above results and the fact that uric acid is widely distributed in the human body at relatively high concentrations indicate that CCPD is a good marker of ClO⁻ generation in vivo .