Hydration studies of commercial plaster of paris in presence of citric acid have been made by using temperature and electrical conductivity measurements. X-ray diffraction studies, Differential Scanning Calorimetry, Spectral studies and Compressive Strength measurements have also been made to understand the mechanism of hydration. Results show that citric acid retards the hydration. It is proposed that citric acid in the form of citrate ions are adsorbed on the plaster surfaces and retards the hydration.