In biomass combustion, hygroscopic and deliquescent salts may cause cold-end corrosion and operational problems, such as deposit buildup and decrease in heat transfer. Calcium chloride is a deliquescent salt that can be found in the cold-end of boilers. In this study, the hygroscopic properties and corrosiveness of CaCl2 in flue gas conditions were studied. The formation of hydrates and the deliquescent properties of CaCl2 were studied with various techniques. The hydrate formation, deliquescence, and water absorption at various temperatures, humidities, and cooling rates were studied using thermogravimetric analysis. A stable flow of water vapor (5-35 vol %) was created with a membrane humidifier with a humidity sensor. Deliquescence and recrystallization of CaCl2 were determined by a two-electrode chronoamperometric setup. The effect of mixtures containing CaCl2, CaCO3, and CaSO4 on deliquescence was also studied. Furthermore, the corrosivity of CaCl2 on mild steel was studied above and below the deliquescence temperature, and the corrosion products were analyzed by means of scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDX). This work revealed that both deliquescence and recrystallization of CaCl2 are of importance when assessing the corrosivity of the salt. When deliquescence occurred, the corrosion rate was substantial, and the corrosion rate was dependent on the ion concentration of the formed solution. Drying or crystallization of the salt solution occurred at 35-42 degrees C higher than the deliquescence temperature with 10-35 vol % H2O. Thus, large variations in the flue gas water vapor concentration will impact the wetting and drying of the salt. The observations presented in this paper give guidelines on how to prevent corrosion caused by deliquescent CaCl2.