Soluble salts are among the main causes of deterioration in wall paintings and the study of their properties is of both theoretical and practical benefit. Calcium nitrate, a deliquescent salt commonly found in wall paintings, was selected to clarify the role played by very hygroscopic salts in wall painting deterioration. The most suitable methods for analysis of calcium nitrate tetrahydrate have been identified, the environmental conditions under which salt activity may occur checked, and some physicochemical parameters suggested for controlling the environment in order to avert such activity. The kinetics of water sorption were studied to identify the various factors which might affect the rate of deliquescence. The variables included the mass of the salt, the presence of other salts, and the kind of support on which the salt is deposited. Thermogravimetry (TG) and differential thermal analysis (DTA) were found to be the most reliable and practical techniques to identify the salt. Calcium nitrate tetrahydrate was found to deliquesce easily and recrystallize under environmental conditions typically prevailing for wall paintings. Of the variables studied, the support had a significant effect on the water sorption kinetics and, most significantly for wall paintings, it was found that deliquescence would occur most rapidly in plaster.