The oxidation of 2,4-dichlorophenoxyacetic acid using ZnO as photocatalyst is investigated. This photodegradation process depends on the mass of semiconductor and on the light intensity, being almost independent of the pH of the solution, except in alkaline media and near to the point of zero charge of the semiconductor. The experimental data show that the photodegradation process follows Langmuir-Hinshelwood kinetics. The role of Fe(II) ions in the yield of the 2,4-D photodegradation is analyzed. It is shown that the addition of a small amount of Fe(II) increases the rate of photodegradation. Also, an increase in the rate of 2,4-D dechlorination is observed, although a lower degree of mineralization is detected in presence of Fe(II) ions than in their absence.