Presently, about 3 million tonnes of phosphogypsum are being generated annually in Spain as by-product from phosphoric acid in a fertilizer factory located in Huelva (southwestern Iberian Peninsula). Phosphate rock from Morocco is used as raw material in this process. Phosphogypsum wastes are stored in a stack containing 100 Mt (approximately 1200 ha of surface) over salt marshes of an estuary formed by the confluence of the Tinto and Odiel rivers, less than I km away from the city centre. A very low proportion of this waste is used to improve fertility of agricultural soils in the area of the Guadalquivir river valley (Seville, SW Spain). The chemical speciation of potentially toxic elements (Ba, Cd, Cu, Ni, Sr, U and Zn) in phosphogypsum and phosphate rock was performed using the modified BCR-sequential extraction procedure, as described by the European Community Bureau of Reference (1999). This study has been done with the main of: (1) evaluate changes in the mobility of metals during the production of phosphoric acid; (2) estimate the amount of mobile metals that can affect the environmental surrounding; and (3) verify the environmentally safe use of phosphogypsum as an amendment to agricultural soils. The main environmental concern associated to phosphoric acid production is that Uranium, a radiotoxic element, is transferred from the non-mobile fraction in the phosphate rock to the bioavailable fraction in phosphogypsum in a rate of 23%. Around 21% of Ba, 6% of Cu and Sr, 5% of Cd and Ni, and 2% of Zn are also contained in the water-soluble phase of the final waste. Considering the total mass of phosphogypsum, the amount of metals easily soluble in water is approximately 6178, 3089, 1931, 579, 232, 193 and 77t for Sr, U, Ba, Zn, Ni, Cu and Cd, respectively. This gives an idea of the pollution potential of this waste. (C) 2007 Elsevier B.V. All rights reserved.