The solubility limits of CaSO4 polymorphs are of critical importance for determining working conditions of thermal desalination processes. This is because fractional fresh water recoveries in these processes are limited by the threshold precipitation limits of calcium sulfate polymorphs. Partial removal of calcium ions from seawater extends the solubility limits of the CaSO4 polymorphs and enables higher water recoveries. However, information on calcium sulfate solubilities in saline solutions is inadequate as available empirical models display significant deviations between solubilities predicted by the different models. The objectives of the present study were to verify the solubility limits of CaSO4 species in seawater concentrates of reduced calcium content. This was achieved by theoretical extension of the Fabuss and Lu's solubility model followed by experimental verification of the theoretical results using XRD analyses to distinguish between the different CaSO4 polymorphs.