<p>In strongly degraded reservoirs, the occurrence of intensive internal supply of phosphate to waters under oxygen-deficient conditions is a problem. The purpose of this study was to evaluate the performance and long-term effectiveness of a properly composed mixture of calcium and iron salts in reducing the intensity of internal supply from the bottom sediments of strongly degraded water reservoirs in terms of their reclamation needs. Experimental studies of phosphate inactivation under anoxic conditions were carried out on undisturbed sediment cores taken from two small retention reservoirs located in SE Poland. The sediments of the tested reservoirs were significantly different in terms of physical and chemical properties. The salt synergy of CaSO₄∙2H₂O, FeCl₃, and CaCO₃ was ideal to inactivate phosphate phosphorus in the bottom sediments of the tested reservoirs. The application of FeCl₃ and CaCO₃ in specific proportions does not lead to a significant increase or decrease in the pH of the water. The decisive influence on the total internal load of phosphorus is the release under dynamic conditions, i.e. during simulated resuspension of sediments and resedimentation of suspended particles. The weaker effectiveness of the applied salt mixture showed in the case of sandy sediments, poor in organic matter, especially humic substances, and significantly hydrated sediments.</p>