An approximation to the knowledge of P forms in soils and sediments is important in agronomic and environmental terms. Sequential chemical extractions have been often used to study the nature of P forms. These methods are based on the selective extraction of operationally defined P fractions by using single extractants in a sequential manner. This can be used as a tool for establishing different “operational” P pools in soils or sediments with a differential ability to release phosphate to the solution. The main objectives of this work were: (i) to study P forms in 17 representative soils from Mediterranean areas and in sediments eroded from these soils by means of two sequential fractionation methods (Ruiz et al., 1997 and Diaz-Espejo et al., 1999), and (ii) identify the relationships of specific P fractions to P release potential in soil and sediments as determined using P sinks (anion exchange resins and paper strip impregnated with Fe oxide). The use of these fractionation schemes allow to discriminate in this group of soil: the more labile P forms (essentially adsorbed P), non-lithogenic Ca phosphates, most of low soluble pedogenic Ca phosphates, lithogenic Ca phosphates, P occluded in poorly crystalline Fe oxides, and P occluded in crystalline Fe oxides. The availability index for plants (Olsen P) was correlated with the P fractions including the most labile P forms: NaOH extractable P + Citrate-bicarbonate extractable P (NaOH-P + CB-P, essentially adsorbed and soluble precipitated Ca phosphates, r = 0.75, P < 0.001) and Ca-EDTA extractable P (Ca-EDTA-P, mainly adsorbed P, r = 0.79, P < 0.001). Amounts of P extracted by anion exchange resins at 1 h and 1 d were significantly correlated with NaOH-P + CB-P (r = 0.73 and r = 0.74, respectively, P < 0.001). Phosphorus fractions significantly affected the desorption kinetics of P release from soil: the ratio of P extracted at 1 h to the maximum extractable using resins was significantly correlated with the Ca-EDTA-P to total P ratio in soil (r = 0.66; P < 0.01). Eroded sediments from studied soils had a higher P concentration and a higher proportion of Fe-related P than original soils. The algal-available P in runoff from these soils (estimated using the Fe oxide impregnated strip - FeO strip) was equivalent to the sum of dissolved reactive P in runoff and fractions including the more labile P fractions in sediment (NaOH-P + CB-P) (Y = X, R2 = 0.85, P < 0.001).On average, 68 % of the P extracted with the FeO Strip was particulate P. An estimate of the long-term P release potential must take in account redox sensitive P forms, in particular P related to poorly crystalline Fe oxides, and mineralization of organic P in sediments. If redox sensitive P and NaOH extractable organic P is considered as potentially releaseble P, the long-term algal-available P might increase by 42 % of the amount estimated using the FeO Strip. References: 1)Ruiz, J.M., A. Delgado, and J. Torrent. 1997. Iron-related phosphorus in overfertilized European soils. 2) J. Environ. Qual. 26:1548-1554. Díaz-Espejo, A., L. Serrano, and J. Toja. 1999. Changes in sediment phosphate composition of seasonal ponds during filling. Hydrobiologia 392:21-28.