The reactivation by hydration with liquid water of partially sulfated fluidized-bed combustion ash from a 165 MWe circulating fluidized-bed combustor was carried out at laboratory scale. The ash particles were separated into four size fractions and each fraction was then hydrated at different temperatures and hydration lengths. The free lime and calcium hydroxide content in the samples were evaluated both before and after hydration. The current results show that the extent of hydration is clearly dependent on hydration time and temperature and affected as well by the particle sizes. There is also evidence that there are additional reactions besides the hydration of CaO occurring in this system.