Two Canadian limestones, Calpo and Luscar, were fully sulphated, and the residues were hydrated with liquid water and steam and then re-sulphated with synthetic flue gas in a thermogravimetric analyzer (TGA). Both sulphated limestones were previously classified as showing uniform sulphation patterns, and it was expected that they would not demonstrate significant reactivation by hydration. However, the current work demonstrates that both spent sorbents can be reactivated by steam hydration, while one of them, Luscar limestone, can also be reactivated by hydration with liquid water, whereas water hydration is less effective with Calpo limestone. Long-term sulphation was employed on the fresh limestones to ensure that the sorbents were fully sulphated to levels typical of full-scale units during the first sulphation. No evidence was found that the SO2 concentration for first sulphation influenced the degree of reactivation, indicating that these sulphation times are sufficient. Total calcium utilization after re-sulphation was markedly improved up to 90% by hydration. Possible explanations for the failure to reactivate these limestones by previous workers may well be that they chose unsuitable hydration conditions and/or that there are wide variations in limestone properties between different batches, even from the same supplier. It is also evident that it may be premature to categorize the sulphation patterns of a given limestone on the basis of limited tests. (C) 2006 Elsevier Ltd. All rights reserved.