Three lots of ash from a small bubbling fluidized bed combustor, each made up of three fractions and two lots of ash from a large industrial scale circulating fluidized bed installation were examined. Limestone sorbents were used in both cases. Ash samples were subjected to phase analysis “as received” and after prehydration at ambient conditions and under pressure. Determinations of the proportion of free CaO were also made for selected samples, including some to which a controlled amount of CaO or silica had been added before hydration. When possible, the thermal effect associated with mixing the samples with an excess of water was measured. It was concluded that during the prehydration process CaO can be liberated from Ca-containing coal ash components but it can also be consumed in reactions with silica or silicates. These processes can affect the exothermicity.