Sethoxydim reacts spontaneously with water resulting in immediate structural changes. Simulation of field conditions of light, moisture, oxygen, pH, and soil and evaporation on siliceous surfaces duplicated this lability. Sethoxydim degradation was enhanced by alkaline conditions, ultraviolet and incandescent light, and adsorption on solid surfaces. No sethoxydim was detected immediately after application to moist soil. Less than 2% extractable sethoxydim was present in dry soil after 24 h.