The paper presents a research covering the stability of phosphorus retention in the bottom sediments, resulting from application of gypsum in anoxic conditions and in slightly acidic environment (pH~5) of solutions. The present work also contains an analysis of possible effects of gypsum application for water from the reservoir, on the basis of selected parameters of overlying water. Undisturbed deposit cores extracted from two research stations on the Solina Reservoir have been a subject to a 10-weeks long exposure in determinate conditions, after application of gypsum. The conducted research indicated that retention capacities of deposits, which become increased after the use of gypsum, will decrease over time (e.g. after winter period). Application of gypsum led to a quite significant increase in concentrations of calcium in solutions. Intense release of iron from the sediments into the solutions and decrease in SO₄^2- content, during the said 10-week exposure in anoxic conditions, indicates the use of iron (III) and sulphates as electron acceptors in process of oxidization of an organic substance.