Chlorimuron-ethyl is relatively stable in water buffered to pH 7.0 and 9.0, but hydrolyzes readily (half-life, 14 d) in water buffered to pH 4.0. In addition, chlorimuron-ethyl photodegrades rapidly and extensively in aqueous solution. The predominant photoproducts are 4-methoxy-6-chloro-2-aminopyrimidine, ethyl 2-aminosulfonylbenzoate, N-(4-methoxy-6-chloropyrimidin-2-yl)methyl urea, and o-benzoic sulfimide (saccharin). A minor deesterified product (chlorimuron) was evident. The decrease in chlorimuron-ethyl concentration in aqueous solutions followed first-order kinetics. The rate of degradation in different types of water followed the order irrigation water > tap water > distilled water. Chlorimuron-ethyl photodegraded in pH 4, 7, and 9 buffer solutions under both UV and sunlight. A faster degradation rate in pH 4.0 buffer solution was observed. Keywords: Chlorimuron-ethyl; aqueous solution; photolysis