Sulfation of limestone-derived particles over periods of many hours may contribute significantly to limestone's utilization in fluidized-bed combustion (FBC). Long-term sulfation may also cause agglomeration. The progress of sulfation in highly sulfated particles of a calcined limestone is investigated. Conversion curves lasting 14-60 h have been obtained in a thermobalance at different SO2 concentrations with particle sizes and temperatures typical of FBC boilers. Also, tests in a tube furnace under very strong sulfation conditions have been performed to elucidate the mechanism responsible for the residual activity of the sorbents and their final fate as fully sulfated solids. Scanning electron microscopy analysis of the highly sulfated samples shows cracks in the external surface of all of the particles, irrespective of their sulfation pattern during their fast sulfation period. Based on this observation, a shrinking-core model is used to fit the thermogravimetric analysis curves with an adjustable cracking constant. This simple model gives a satisfactory fit up to conversions close to the full sulfation of the particle. However, in samples allowed to agglomerate, the onset of agglomeration must produce a drastic change in the effective particle size; this explains deviations of the model from results obtained with real ashes in very long sulfation tests.