Addition of phosphate to soils is a common agricultural practice. As phosphate anions are known to adsorb strongly on many soil minerals, the presence of important amounts of phosphorus fertilizers is likely to cause a noticeable decrease in the adsorbing properties of soils with respect to other anions, e.g., anionic pesticides. This chapter studies the competitive adsorption of the anionic herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) and phosphate on various soils in southwestern Spain. This herbicide was chosen because it was one of those recommended in EEC Directive 79/831 for the ring test on adsorption-desorption of chemicals in soils, conducted in 1989 with the participation of the authors of this chapter together with other European laboratories. 1