Experiments were made on the effects of pH, phosphate concentration, particle size, iron, and organic matter on the adsorption of phosphate by estuarine bottom deposits in as natural a state as possible. Measurements of the ability of the silt to adsorb phosphate were made by isotope dilution, using 32P. The silt was separated into four fractions by sedimentation. The ability of silts to adsorb phosphate was directly related to the ratio of their contents of iron to organic matter. Organic matter depressed phosphate adsorption. Suspensions of silts (2.99g/1250ml) adsorbed 80-90 per cent. of the phosphate in solutions containing 0.55 to 2.55 mg of phosphate P. This suggested that adsorption caused estuarine silts to trap phosphates during run-off periods.