The rate of transformation from CaO to Ca(OH)2 during the slaking process is influenced by a number of factors. In this investigation the effects of common ions normally encountered in the slaking water were investigated. It was found that chloride and nitrate ions enhanced the slaking rate, while carbonate, sulphate and phosphate ions retarded the hydration of various limes used in the investigation. The increases and decreases in the degree of hydration correlate with the solubilities of the anions forming more soluble compounds than Ca(OH)2, as well as the solubility constants of the anions that formed less soluble compounds than the hydrated lime. The geological origin of the lime also influenced the slaking of the limes.