There is little information about pesticide photostability on plants, especially when considering the effects of the formulation. We evaluated the photostability of a herbicide, mesotrione, on wax films. These surfaces are good systems to mimic the outer layer of the leaf. Within the range of recommended agricultural rates, pure mesotrione half-life on cuticular wax films was between 100 and 160 min. Formulated, the phototransformation rate was multiplied by a factor of 4.8. We assume that the acceleration is mainly due to the surfactants, agents that allow a better spreading of the active ingredient at the leaf surface. Since mesotrione photolysis is a fast process on wax films, we can assume that this process would be significant in the field after treatment.