The second Fick's law of diffusion, considering boundary conditions that at both slab faces the concentration of herbicide is equal to zero (sink conditions), has been adequate to describe our kinetic data obtained from experiments on 2,4 -dichlorophenoxyacetic acid, (2,4-D) released from lignin-based formulations in a water static bath system. However, the same model proved to be invalid in describing the experimental data obtained with ametryn (2-ethylamino-4-isopropylamino-6-methylthio-1,3,5-triazine) and diuron (3-[3,4-dichlorophenyl]-1,1-dimethylurea) formulations in a water dynamic bath system. For ametryn and diuron formulations, because of the lower aqueous solubility of these herbicides, it was necessary to model a stagnant film at the formulation surface to describe better the release kinetics because the model incorporating sink conditions is insufficient. This study presents a new mathematical modeling of experimental data obtained with 2,4-D formulations in a water static bath system. The new model incorporates a stagnant film as the boundary condition at the formulation surface, and its diffusion coefficient value is more precise than the one estimated by the model employing sink conditions.