X-ray diffraction, infrared spectroscopy, thermal analysis, scanning electron microscopy, and particle size analysis were used to characterize the product of the thiosulfate process. It was found to consist of a mixture of a solid solution of calcium sulfate in calcium sulfite hemihydrate, calcium sulfate dihydrate, and calcium carbonate. Reactions taking place on heating the byproduct were identified in oxidizing and reducing atmospheres. Pure calcium sulfite hemihydrate as well as solid solutions of CaSO4 in CaSO3.(1/2)H2O with different sulfate concentrations were synthesized in the laboratory. Thermal analyses of pure solid solutions proved that the higher the sulfate content, the higher is the dehydration temperature. Accordingly, taking SO42- in solution increases the stability of the byproduct. Above the solubility limit, thermal analysis detected the formation of gypsum which precipitated as a second phase.