The object of the investigation was the retardation of gypsum plasters with citric acid. For that purpose measurements were carried out to get information about the mechanism of set retardation and the influence of citric acid on the microstructure and properties of gypsum plasters. Citric acid acts as a retarder by forming citrate. The citrate affects the nucleation and the crystal growth of gypsum by adsorption. That entails the retardation of the hydration period and the habit modification. The influence on the microstructure was investigated by means of the quantitative image analyses and by X-ray methods. Citric acid changes the properties of fresh and hardened mortars. The volume of setting and hardening gypsum plaster passes through a minimum. Increasing citric acid additions raise the minimum and decrease the following expansion. Strength formation was measured by binding and compressive strength tests and by Brinell hardness determination. With increasing citric acid addition, paralleled by higher humidities, a strong decrease in strength can be observed. In addition, the influence of citric acid on the creep behavior of hardened gypsum was examined.