This article reports the general characteristics of the sediments of two Argentinean reservoirs, which are used for water supply. The chemical composition, granulometry, and specific surface area are presented together with a study of total phosphate concentration and phosphate association by combining sequential extraction and microprobe analysis. In general, the sediments of both reservoirs have similar characteristics. Sequential extraction reveals that the main P fractions in the studied sediments are Ca-bound phosphate in river mouths and Fe-bound phosphate in the rest of the reservoir stations. Microprobe analysis appears to be an important complementary technique to sequential extraction. Combined with chemical fractionation and specific surface area measurements, these analyses indicate that Ca-bound phosphate is mainly distributed within grains or particles highly concentrated in Ca and P, whereas Fe-bound phosphate is rather homogeneously distributed in the sediments at the surface of fine particles. Microprobe analyses also revealed an important coating of phyllosilicate surfaces with Fe (hydr)oxides, which explains the good correlation found between Fe-bound phosphate, clay fraction, and specific surface area. The role of sediments as a potential source of phosphate is discussed in terms of P association in the solid phase and dissolved oxygen concentration profiles in the water column.