The accumulation of calcium and sulfate ions in a process stream of a closed papermaking system may cause calcium sulfate scale formation. This study presents measurements of calcium sulfate precipitation conditions and a thermodynamics-based model for the prediction of such precipitation conditions in the papermaking white water at 50degreesC. The precipitates were found to have the structure (CaSO4.2H(2)O). The model calculations were compared with experimental data and found to be in good agreement.