The degradation of clopyralid with the homogeneous and heterogeneous Fenton process was carried out under mild reaction conditions in aqueous solution. A complete degradation of clopyralid was obtained with the homogeneous Fenton reaction. Therefore, the influence of several parameters such as the initial concentrations of the catalyst, clopyralid, and H2O2 as well as the initial pH value on the homogeneous reaction was investigated. Several reaction intermediates and end products could be identified and allowed one to propose a degradation pathway of clopyralid. The heterogeneous degradation of clopyralid could only be achieved in the presence of HCl. Under these conditions, iron ions were detected in the reaction suspension, which indicates that the reaction is not heterogeneously but rather homogeneously catalyzed.