The report describes an experimental investigation of the hydration process of spent bed material (with a calcium/sulfur ratio of 3) from an atmospheric fluidized bed combustor for maximum temperature, rate of temperature rise, and controllability of temperature rise with various amounts of water. Tap water was supplied through a rainfall simulating device. For the volume ration of the spent bed material to water larger than 1.2, the temperature of the spent bed material rose to 77 C; at ratios less than 1.2, the maximum temperature of the material was 132 C. Rates of temperature rise in the uncontrolled hydration process were higher than those in the controlled processes. The hydration process expanded the volume of the spent bed material faster than by moisture in the atmosphere when the material was exposed to the ambient atmosphere.