The Odiel salt marshes (Marismas del Odiel) are an important nature area declared a Biosphere Reserve, but they are greatly affected by pollution from the Odiel River. Surface sediments from this area were analysed using the latest version of the BCR sequential extraction procedure to determine the fractionation of As, Cd, Cr, Cu, Fe, Mn, Ni, Pb and Zn among four geochemical phases (acid-soluble, reducible, oxidisable and residual). The total content of each of the metals and As was also determined. The results showed high concentrations of As, Cd, Cu, Pb and Zn, with maximums of 791 mg kg(-1) of As, 8.5 mg kg(-1) of Cd, 2,740 mg kg(-1) of Cu, 1,580 mg kg(-1) of Pb and 3,920 mg kg(-1) of Zn. The concentrations of Cr, Mn and Ni were low since there are no sources of pollution by them in the area. A comparison of the metal and As levels with the sediment quality guidelines showed that the pollution is sufficient to produce noxious effects in aquatic organisms in most of the Odiel salt marshes. Based on the chemical distribution of the elements, it was found that Cd and Zn were the most mobile (i.e., elements that can pass easily into the water under changing environmental conditions). However, Cr, Fe, Ni and As were present in the greatest percentages in the residual fraction, which implies that these elements are strongly linked to the sediments.