Two aspects of the possible reactivation of fluidized bed combustion (FBC) residues are considered. One is the behaviour of aluminosulfates such as ettringite, which may form when excess water is used in hydration methods designed to reactivate the lime in the FBC residues. The second is the behaviour of compounds such as calcium aluminates, silicates or ferrites (＇other calcium compounds＇ or OCC) that are produced by reactions in the fluidized bed. In both cases, the possibility of recovering the CaO contained in these materials for retention of SO2 when they are reintroduced into a combustor is assessed. It was found that ettringite is an excellent sulfur sorbent under FBC conditions, with reactivity superior to normal limestones. The CaO moiety in the OCC is not easily reactivated by hydration. Unhydrated, they do react in a sulfating atmosphere like that of the combustor, but their performance as sorbents is much poorer than limestone and varies with the nature of the compound.