5-Chlorouracil, a potantially hazardous substance formed during chlorination of natural waters, is subject to photolysis in solution when irradiated by naturally occurring wavelengths of ultraviolet light. The undissociated species of 5-chlorouracil does not degrade in solution even when irradiated by shorter ultraviolet wavelengths than penetrate the earth's atmosphere, but the monoanion, which predominates above pH 8, disappears at a first order rate when irradiated by 290–320 nm light. Variations in pH and temperature have pronounced effects on the rate of photolysis., with higher pH and temperature resulting in more rapid breakdown. The photolytic half-life of 5-chlorouracil could be as low as two days under some conditions of pH, temperature, transparency, depth, and illumination, but in most cases the rate of degradation would be considerably slower.