With high pressure mercury lamp(HPML) , xenon lamp and sunlight as light sources, the photodegradation of mefenacet in five different water samples, three different organic solvents were investigated. Under HPML, six different pH value buffers, NO3-, NO2-, X-, acetone, four surfactants and four herbicides were added to the solution of mefenacet respectively to observe the effect on the photodegradation. The results indicated that the photolysis of mefenacet in water is mainly direct photodegradation, and it...